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unllkeiv to be of value and often causes un- necessan anxlen•: Nutrition is better assessed using skmfbld calipers (whleh are also cheaperand more portable than weighing scales: to measure dsrect)v the thickness of subcutaneous fat.' Accurate height measurement (supme length inn infants under 2}•ears) is a sensitive guide to child health.' Growth velocita (caltulated from repated' masurcments of Ihetght at in tervals) iepresents the currenr dpnamics ofl growth much better than ~ a single measurement,: w•hicti reflects previous grow•tli, Regular, accurate measurement of children can idenuf.• those who would benefit from i medical, sociall or educational intervention.' Many height measurements in hospital and the communm are inaccurate and arusacading because of careless techniques and inadequate apparatus. Suitabli•accurate, cheap, and pottable apparatus is now widely availabVe for use in primary care, and' measuring techniques eliminating postural drops and' positional errors arc radil?,• larnt by mou- vated staff. Supine length in children under 2 years lan generally be measured accurately with the help of an assistant. Collected accurate growth (height) ~ data in children have important benefits beeond'ehose to the individual~-as an index of the health of a population or a subgroup (for example, ethnic group or social class). British data are not available and would be valuable. btanv who care for children lack the skill to measure them accurately, plot measurements on a growth chart, and interpretthe data obtainedi As the repon states; such understanding is essential for growth' monitoring. More must be done to make those who look after children aware of! the need to measure height accurately and regularhthroughoutchildhood and'to train them to do so, CHRISTOPHER J.H'3;ELKAR Detunmem of childLfe and Healtti. U m.-ervryof Edm bu rgh. EGmtiurah EH91 u•St' I I PdhatL. CYild heahb, . sunvrdlaoce. B0 AfN.J,19a9i299;135112. fbDecembeul 2Hall D A1B: ed: Hma6 for a1JcAildren. tIr nym ol'~?~ *+atsnr pam-an ekld 4e4hsrxdb+re. Odasd: Oafdrd' UnnernnPress, 1989'. 3Tanna J tit, Q'hstehouse RH. Rnned ssardsnds fdr.tnceps andd subscarwiirsundards.m Briush chitdren. AnA.DuC&I! 1975:5a.14.-1. t Tsnon J,\L. Fou+a mo wan.: 2odd ed; 7Carc: Casrknwd Publra- uo ny19E9. S1R--Health Far A71 Children, discussed bv Dr Leon Polnav''and'Dr D Is4 B Hall,' is the result iof a working patTV, set up by groups represenung' paediatriciansand general pncutsoners, neithenof, wwhom is' disinterested. The British Paediatric Assw:iauon suggested'some years ago that senior clinical medical officers in the child'health'service should 6ereplaced by "communiq• paediatricians"' who would work partltimc as paediatricians in the hospirtal and'w•ould take part in the on call duty roster. Similarly, much of the interest in taking over child health surveillance by general praeti- uoners has been tied to the proposal that extra payments w•ould' be made for such a service.. Practinonerswt.o have a real interest in thiswork provide such a sen^ice already forpatients on their lists. Bodies that actually represent the medicall officers who w•ork in the chiltl'healih:sen•iee were not invited to join t3tc working parrv. Child health surveillance requires a different outlook from clinical medicine, and it is not asy for clinicians whose whole training has Fieen directed to the diagnosisand treatment of disease tostop.thinking in such ter7rtsand abandonrheir prescription pads. Clinicians are not the most appropriate group to adsvsee onn aeliild healthi service that thm' do not fuliv understand. Developmental assessmem and ehild healtli surveillance w•ere pioneered bp the former child! health group of the Societv..'of MediaJ Officers of' HealttiL which started running full trme training courses of six weeks' duration for doctors some 30 vears ago. !n the ari} 1970s when the Facult v of Communirx- Medicine was formed eommunitv health doctors were not ehgible for: membership. Fortunatdk-, a number of medical schools started to run tratning courses in child deeeiopment':to fill the need that resulted. There w•as• hown•er, no orgamsauom or body monitoring the standard' or contenl of those courses.: which caried'widelv. Following the formation of' the Faeulrc of Communitv' Medicine restdual members of the Soeietv oG Communit}• Medicine sought toapro- mote the interests of communin.• health as well as eotnmunin- medicine. ln 1988 the society (which has since changed its name to the Soeiery of Public . Health) w•as mstrumental imestablishmg a new Facultv of Gommunin• Health to produce sylla- buses• set standards, and4ppoint examiners. ]n future, membership of the Faculty of Communin• Health should be evidence of eligibility for pasts as senior clinical medical affscer or as consuttann in aommunin• child health-more appropriate to the needs of the clients and' of the child health and education services than "community paedia- tricians." We hope that this faculn• will'providt training for general practitioners in child health sun•eil- iance and that appropriate diplornas will be established. S- of PuhYK Heahh. Loodor, WI I:aDE P A GARDHER J'SROBERTSON I Po1oa.:U..Gildhealih su-ildance.Bi.MrdJ.1989'~299•13512. ;2 December., 2 HaffiDMB. Cdild heallh surveillancr. B. AtedJ 19g9:W9:1353-3.. f2 Descmher.) Lee, F N.: assive smoking and cardiorespiratory health in Scotland SrR,-In an earlier letter' I claimed that misclas- sitiation of active smoking statc can explain the fact that F4rDavid J Hole and his collagues' found weak positive associations between passive smok- ing and a number of indiators of cardiorespintor}• health in the Scottish prospective stud}'. In their reply Mr Hole and colleagues prescnted'calcula- tions to justify their view: that the effect of misclassification is to produce "only small biases in the relative risk for passive smokcrs„" with the reported relative risk "wcll in excess"' of that produced by' this form of bias.' Uhfortunatel}', these calculations are grossly in error and therefore highly misicading. The error lies in basing aloulations on results for men and women combined withounadjustment for sex. Table I of the original paper° shows a clear TABLE1-"Obsrrvrd" rrlariar risks for passrvr smoking for varwng dental rares of fmoking+ Relau.c nsks for passivc smoking . Ratrof denul. ~~.~bsned (M1.~ Aien Women Ad,ustedt Umdlusted$ 2~ 1~74 1~-25~ 140 1~12 A 1'95'. 1~~42~. 11 58 1~18 6~ 2-06: 1`54 1!70 1~20 8 2~ 11: -63 L~iB 142~~. )0: 2~I5'. 1~~70 1 h9/~ 1.23~. •' Assummg "truc "'relau- nsks of I h0 for passtve smok.g.nd 20-0 fw acuvr smoktng. TAdtustadfdrseaustng warhis Ni\•1;\,+N;'., ~-hac N, and S,+rc the oDxrced numtrn of exprssed. and unexpned sabreen. Thus.o.a conxnauve appraaumauon io the. vue ad,usted frgure, Swhtch unnot bc caiculaN pre<txll lsom the dau prorud2d, b, Hodr rr ndi jAsprrnMHolerra':' association between the smoking habits of the index case and the cohabtttt, with the coneordance. (cross product ) ratio being:2-32 for men and'2 19 forwomen. Amappropnate esumate:of.the.eoncor- dance ratio for the sexes combtned with sex adiustment bv the Mantel Haensaei prtxedure"ss 2_'5. !f: inappropnatelc, the concordance ratio is alculated from the pooled data, a much lower figure of 1-29 tis obtained, and this masks mosl of the true association, This is importanl' because it an readii.v , be shown that the concordance ratio pror•ides the upper limit to the extenl of the observed relauve riskSrom passtve smoking due to misclassifiauon of smoking habtt (assuming a true relative risk of I`0). Table I shows that when correctlv calculated the obsened relative risk can far exceed the value of 1-20 stated b• MF Hole and hrs,collagues to be "the largest risk to be among passive smokers due to this form of bias."' The question arises as to the extent that' this source oflbias can explain all the reponed relaure risks for acttve.and passive smoking seen tn~.the Scottish stud}•. Table II'gives some insight into this question, showing "obsen•edT"and "true" rel5tive risks assuming a 4% denial of smoking; a figure consistent with data from mam• studies of the issue.' Comparing the "Observed" relativerisks of active and passive smoking with those glven in, TABLE rt-"Obser.,ed" relarsrxntk's forpcsnxandaranxnnokmg fortwy.mg "true" relatr.x.ntks fm sen~r rmaking' "Tirur" sliuve- nsks •'Obxrvedl' relnuvo niL, Pis- snaken Acuvc. smoken Passivc. srnokrn A:usr anokers 1 30~~ 1~70 9~~17. I 20~. 1~58 7-g8'. I 10'~ 1~39 5:,62~ I 5 143 3"63 I. 3 1~'13 2`S0~ I 2. I~OT~ t~gl • Assummg~'4 ab smok'rrs drmsmolung. Resulis arc for srxrs . romburcdadtwssed for ses as in ubk I. table V1I of the original papcr shows that there iss no problem whatsoever ia reconciling the data with the bias hypothesis for moscof the cardiorespira- tory endpoints.,Forcxample,,rclative risks of 3'77 (active) and' 121 (passive) for HpJxrscereuon are both verp close to the values given in table II for a "true:" acuve risk of5 (I -23 and 3-63/ respectiveh•). Only two endpoints deserve special comment. The first isdeath from lung cancer, for which risks of110•64 (active) and 2-41 (passive) were observed. The confidence mten'al for the risk with passive smoking was enormously wide (045 1o 12-83), and the point estimate of risk was higher tFian that in any of over20 other, largcr,studies on the issue.' I have claimed elsewhere that' miu7assi5cation of activc smoking state can explain the azeragee relative risk for, passive smoking of about 1-3-1 5 seen in epidemiological studies.' I retain this taew but have never stated that it explatned'1he figure in ehe Scottish studr, of'2-41, to which chance hasy clearlv contnbuted'substanuallp,. The other endpoint is ischaemic heart disease, fon which risks of 227 (active ) and 2-01 (passivel were observed. Although the risk w•ith~ passtvee smoking issignihcant (959beonhdence interval 121 to 3•35) and the lower confidence limit is slightly above the biasaxpectedy I do not:hnd this convincing evidence of a true effect of passtre smoking. ThisispartlN because the signifiancelevel is not high, bearing,in.mindthe number of endpoints stud)ed;, and part)v because d,e point estimate ofrerclativertskf for passive smoking tss difficult to reconcile with that for active smoking. 6earingin mind that smokers havee much'~hi¢her aetivrandpassive exposure to the consutuents.oi'. smoke, in thce ftsrm of both marnsueamand'stdestream smoke, than do passavel},cxposed non, smokers. More evid:ence is cleariv needed here., The American Cancer Srxrervmilhon person studc BA1J VOLUME 300 13 JA~;VARY 1990 120 ~

accumulated 153 deaths from lung cancer and many thousands of deaths from ischaeauc hean fLsease in oon-smokers,. The effect of passive smoking on lung cancer has been looked mto."It is a pttv that its effect on tschaemsc heart disease has -+ot. , PETER N LEE ` :vSist2 5 D A I l.ee P•.'. Pasnve smdung aod ;eud,oresy+uawntira1tL un.Sa:- land:.. Br.Ned J1989299:712.. (16september.' 2Ho& Dl.Gdln CR. ChopraC• Ha.nMrnc V M_ Pasavel smoksry arrd ordwrnpuuorytnlt6: m a genera/ pupulaunotn tne .eau. of Srouand. B:•.HedJ 1969'.-'99:123~:,.~,y;dttgu~i. 3Hole Dl. Gdiu CR l CLopa:C. Hawdioro< t'AS. Psuvr. smaktogand cardurespnrory hdrL m Scodard. Br.MdJ19g9-•29PI I0V 1. 4 Stamcl. N. Haenvel R'. Suusud aapecn ol tte am1.+•su of dau irom tevospreuve srudtn a.Lseaar..7 •V.W Cawrer.l- ..1959; . :71946. 5!se PN. Passive. serokmgandIunB, caocer: faci or 6cnon? 1n: B+era. Cl1 Coumou Y. Gonen. M, eds. P,r.ew l.rre ef uda. . pyafuv. Amasudim:.EUrv,er, 19g9i 119-26. 6Gaaninkef L. Tlme trends m 1.4 - . moruhry amoog rwn-smoken ard. a nore on psave snnakrng- J A'al 6anen lw, 198'd:66:1061b:' AurHORS' REPLt',-Our calculations are neither incorrect nor misleading. .41r Lee is attempting to show how large a bias can be introduced into esumates of rdative risk for passive smokers due to active smokers misclassifying themselves as non- smokers. In doing so he has produced'biases that are excessive because we can show his assumptions are false. His main tnistake has been to assume tliat the "true" reliuve risk for lung cancer is the same for male and femaie smokers (his uble I): Also, although thrextent of smoking denial focour study is nouknown„we can put an upper boundary on it. Our original study estimated the relative risk of 1ung cancer among active smokers as 8 49 for men and 3-33 for women.' Table I shows, under Mr Lee's assumptions, thatl 'bbsen•ed" rel9tive risks fnr active smokers would be lirger forwomen than ~ n. This is incompatible not only with what we .ce observed but also with all other reports we know of. Thus his assumption that the same "true" relative risk holdk for both men and women TABLE tr"Obserxd"redarme riik's foractive andd passiue smokers for var)zng denial'rases of rmoking• RrJati.c nsks for Relauve risksfor Ratr ofactive.smoking.f panwe smokung . denial (Y.), . NSm G'omen \fcatL'omeo 1 10.34 16~20~ 1153 1.15 2 6..90. 13•3? It71 1~25~ 3~. 5`Ii 11-53. 1!S"~ L34 4' 4~10~ 9.99~~ 1'95~ 1-42~ 6~ 2YSfr~. 7. g9~~ 2~~06 134 1 : 2-20. 6~48~~ 2:*11 10 1~76~ 5~45~~ 2`15 I~70~ is untenable. Also, if we accept Mr Lee's aheorrb- cal range of possibiliues for the rates of denial of cigarette smoking then the outcomes become even more unlikely. For each rate of denul ofl4?/e and over suggested'by Mr Lec the relative risk for malt acvve smokers is progressively well below that observed' in our study (table I). Above a denial rate of 8% the "observed" relative risk for male passive smokers exceeds that for active smokers. Our data are, however, compacible with dettial, rates of up to 2% and a "true" relative risk of 4 for female smokers. Mr Lee questums the extent to which tnisclasl sificauon can explain all the reported relative risks for active and passive smoking seen in our study:. Table II shows the relative risks for active smokers foundi in our study for each endpoint and the "true" relative risks with which these are com, patible, assuming a rate of denial of smoking: of 2%. For example, the relative risks for all causes of death assoeiated'with active smoking are 1-85 for, men and 1F87 for women. These figures are compatible with a°true" relative risk of 2, given 9 denial nte of 2%. The figure of 5 that Mr Lee quotes in his letter may be appropriate for some of the endpoints used but certauJy not forall. The final' two columns of' table 11 show the passive smoking relativrrisks foundIin our study for each of the endpoints compared with those that could have occurred through the type of bias Mr Lee attributes to our, study. In particular, the differences are quite noticeable for the four cate- gories of mortality. Thus tnisda.9si6cauon can bias estimates of.relirive risk for passive smokers that use assumptions compatible with our estimates for active smokers. Thesizc of thesrbiasa does not, however, explain our passive smoking results. What is striking about our results is their consistency across a wide range of endpoints in addition: to~ lung cancer and especially for ischaettric heart disease. This is supported by our findings of' a dose-response relation for each of these. Even though bir Lee reaffirms his view that misclassibcaton of'aetive smoking state can explain the average risk of lung cancer with passive smok-mg, we welcome his implication that the effect of passive smoking on ischaemic hean disease is worth further investigation. DAVID f HOLE CHARLFSR GrLL]S wrss of S. m Wd Ca.ea Sw.dtama Lnrt, RudsiO HYnpul, G6sgo.G209N8 Depanmrar a Epd-ulogy,.. l.'m.resm ot.lt,eh,gan. MlrLryu. limted'6uus VICTOR M'HAWTHORh-E I Hok Dl, Gd16 CR. Chopn C, Ha+ehome V M. Paavvr smek,og andcaro,orespu-nory holdt or a grural populauao rn the .eu of Scouaod.. Br.Hed J4989L9i: [23-T. (12 Augusu ) l `.4ssuming '•uve" rclauve nsksof 1-0 for passrve smokusg and 20 for acu.c smokvsg. ." Tliis correspondentt is now closed. - ED, BJK,7. TABLE tt -ReLati;•e: risks ft>und in study tompared ~u•ith"'tr7ue"'re/dave riskr.Jor aative smakens assd "observed" relanae risks forpassrve smaken• . Endpotnt Study findtng. Itiocted:phtcgrn Persvcm phieg:n 4!03.. 4'33 Clyspnora Hvpeesecresron Angnu .ltalor abnonnal Ck:cirocardMsgram 1165 2•95 2,13' 1157 ' 31t CaWSes of dcatb 1185 . `nuc Eon duns< I 136 mg cancu .sIS aasea of dut4 rrutrd w smotung 8I 49 I190 Acticcamokcs Pissive smokers Men. Women Bothsous~. "Trui " "True^ ••Observed": relauvc Study rNaove Study rdiu.c nsk fiM1ag risk findtng nsk 6.0 3925-0 1-34 1114 60 3r935•0 1~19: 1114 1.9 1~3714 : 1~091103~ 5r0 4~15 5.0 121 IIi3' 2-7 I-4-t 1-51~11 1105. 1-8 0_92 1.1; 127. 11022~0 1-97. 2.01~27 1l0e'. 3-0 2-99. 3-0: 2'01 1-0720.0 3-33 4-0. 2 41 I•263-0 2~453•0 1-30' I'07 ' Assumm8 2% of smokers: dcnvsmokSng_ TiheresWrse for botif sczn- cnmbsned: haecw been adtusredd for. se:= ususg.vesgbtsN,Np(N ,/N_ ). -here N;,and :N, ase otiserved numbetss of exposed and unespore.d zuliieets. I/ Congenital malforrnations. StR,-In her editorial on congenital'malformauons Professor Eva Alberman cotrtmentson the excess rate of deaths from malformations, particularly neural tube defects, in infants of mothers born in Pakistan.' Ih~the studies referred:taonlv pennatal deaths were considered. Many neural rube defects in this country are now detected by prenatal screening programmes, and women may, opt for termmauon of the pregnancy when found to have an affected fetus,' so thesc studies may not reflect the true incidence of neural tube defects. Asian women iend to book later for their antenatal care,' and this may acrounrfor the high crontribuuon of neural tube defects to perihatal mortality: second trimester screening would be available to a relatively smaller proportion of Asian women. Furthermore, they may find termination of pregnancy unaccept- able on religious grounds.' We have investigated the overall incidence of neural rube defects by ascertaining all those affected fetuses detected by..' prenatal screening with ulvasonography, as well as all,those found in the perirutal perood': We have alto tried to determine factors that may be impor- tant in explaining any racial differences in the incidencc. We reviewed the materniry ultrasonognpby department records, neonatal land labour tepsters, an&necropsy,reports from January 1980 until the end' of' December 1987 in one district general hospital to ascertain all fetuses, stillbirths, and neonates with a neural tube defect. The maternal notes were then irt.spected'to determine the date of booking for antenatal care, if'and w•hen an ultra- sound scan was performed, and whether a termina- tion of pregnancy was offered. In the Pakistani population there were 111 neural rube defects in a total of 3777 births (2-91 per 1000); there were 32 neural rube defects in 28 834 births to white womI per I000) (nble). Incide+tuof /rearal'aubr defecuu in,fetr.ses and babies:of' wkueand Pakistani tuomen, 1980-7 R'tsice Palustant women vmen Detectcd bv.rvucine ultra.ouad son 17 5Pregrumcyactmmal 17. 4 Ptegnaocy.conunurd . I Not deereted bv rnutine san IS6' Scaa notavailiblr 12. 3 Nor derened by sean 2 1 Booked tooaaee forscaa I I Did not atrcnd for scan I Toul neural nnbc defecn 3211' Toul binhs: 29 834 377 7. lnndcncr= per.1000 binhs 111 291 Routine examination with ultrasound was intro- duced onlyin 19841and hence was not available to many of the women included in this study. The incidence of neural tube defects in the Pakistani population wasaignificuttly higher than that in the white population (p=0j-013; Fisher's exact two tailed test; relative risk 2•62, 95% confidence interval 1-19 to 5•34): One woman in each group booked tooo late for routine prenatal Iscxreening,:and one Pakistani woman failed to attend'for the scan. Thesenumbetsaresmall, but it is of note that the mean gestation arwhich these women booked was 18-2 weeks in the Pakistani group as compared with ~ 14-3 weeks in the white group. Six of the Ill Asian babies with neural' tutie defects were born to women witha consanguineous marriage. We have shown thatt there is arll increased incidence of neural tube defects in the Pakistani population, with late booking and reluctance to terminate an, affected pregnancvy contributusg minimallv to the increased incidence found in pennatal deaths. Changes in customs are difficult to encourage but may well occur spontaneously as BMJ VOLUME 300 13jANUARY 1990 121

accumulated 153 deaths from lung cancer and many thousands uf deaths from tschaemtc heart Htsaase us non.smokers. The effect of passive smoking on lune cancer has beeniooked Into.' It is a ptty that its effect on ischaemtc heart dssease has no[. , RETER'N LEE mm~, ~rrey.5;~i1 S DA I! Lee PY. Pzsu- nmoi,utt and.ard,orespte+sor, nealtN.tn S:ot• Idnd: Br.11rd J, 19r9:.99.74;. ! 16 S<oremoer.? 2Holc Dl. GJlis CR. Chopn C. Huwnor.nev.M . Pasn,ve smoksng and cuwornptnwnoealsn tn s: ecnerat pupusu,oo ustAe wt of Seottand. Be.N<d J1969:.99:~ 23-7: t 1: Aweuss•, 3 HoteDI.GJL.CR.UopnC.HavqeorneV\t. Pasuvesmolung; and ordipeesprtaionv.hnlshl m, Scmlusd..Br N<d J:19A9~'99• n0dl. +.tlanml \, Haensrcl fr'. Suusuul saprcu o( tne analvua o( aau Irom rerro.pecuvr stwLn oi!dtseasr. j..\a/ Cwre /nu 1959; 5tse PN. Pasuve amokme.and lunt cancer• fan or 6cuun? 7n:. Biev. Cl: Counou Y,, Go-res 51. Ns. P<nnv! .ne ef.e,im.. av p.a~u+y: Amsserafon: Else•1er. 1989 . I 19• ]s: 6Garnnkel L.. Time uends us lung can<er mortaL, among.non~ smoren andsnwe oo W- smoiwg: J:Nmi Ca+<rrhus 19a1:66:I061+E:- ALR}i6RS' REPLl',-Ot1r calculations are neither incorrect nor tnisleading. Ati- Lee is attempting to show how large a bias can be introduced into estimatesaf relative risk for passive smokers due to actsve smokers misclassifN•ing themseives as non- smokers. In doing so he has produced biases that are excess4`re.because we:ranSDow his assumptions are false. His main austake has been to assume that the "true" «lauve risk for lung cancer is the same for male and' female smokers (his table I). Also, alihough the extent ofsmoking denial for our stud'y is not known, we can put an upper boundary on it. Our origittal study esumated the relative risk of lung cancer among acnve smokers as 5 49 for men and 3-33 for wromen.' Table I shows, under Atr Lee's assumptions, that "abscr>tied" relative risks for active smokers would be larger for women Ihan men. This is incompatible noronly with what we sare observed 5ur also with all other reports we know of. Thus his assumption that the same -true" relative risk holdk for both men and women TASLEt-"Ob~s.rrt:ed" re7ativerisies for aetive and, paniae tvsokrrs fo.va>nzng denial'rau.s afsmowtn;~.' Reianve:. nsks ior R<Fauve. nsks for R- of uuvc smoKSng , passtre smokmg demal Mo 0.'amea]ten Qnmen~ 1 10~.3d 16•.20 1.53. 1~15 ?~ 6.90i 13,4= 1~74 1~.:5~ 3 5,1a I1- 5 3 !~57. 1~3a i 1~.101 9~~99~ 1.9s~~ 1~..2' 6 2.35~ '"9~ 2 ~06~ 1151. 8~~. 2'20, 6-48 ?-:1 1163~~. 10 1-76~ 5-15 ?`l9. 1170 '.{ssumtng."uve" rctauve.rtsks.of 10 for passive smoking and20:foracuvesmoung. is untenable. Also, if we accept Mr Lee's tbeoren- al range of'posstbdiites for the rates of denial of cigarette smoking then the outcomes become even more unlikely. For each rate of demal of 4% and' over suggested by Mr Lee thcrelative ruk for male active smokers is prngresatvely well belbw thar observed in our study (table 1). Above a denial rate of 8% the "observed!" relative risk for male pusive smokers exceeds that for active smokers. Our data arc, however, compatible with detl]al ntes dup to 2% and a"ttve" telative risk of 4 fbr female smokers. Mr Lee quesnons the extent to which misclas- sificauon can explain all the reported relauve risks for active and passive smoking seen in our study. Table II showsahe relat9ve risks for active smokers found in our srudy for each endpoint and the "true" relative risks with whicb these are com- pauble, assuming a rate of denial of smoking of 2%. Fore:ample, the relative risks fbr all causes of death associated with acnve smoking are 1-85 for men and 1•87 for women, These figures are compatible with a "true" relative risk ofQ, given a denial rate of 2%. The figure of 5 that Mr Lee quotes u5 his letter may'be appropriate for some of the endpoints used but certainly not fbr all. The final two colitmns of table II show the passive smoking relative risks found in our study for each of the endpoints compared with those that could havetxcurred'through the type of bias Mr Lee attributes to our study. In particular„ the differenees are quite noticeable for the four carte- gories oCmorulity. Thus mtsclassinaauoo can oias estimatesof relauve risk for passive smokers tbat use assumptions compatible with our estimates for acuve smokers. The size of these biases does not, howcvct, explain our passive smoking restilts., What is striking about our results is their consistency across a wide range of endpoints us addition m lung cartcer and especialJy for ischaemic heart disease. This is supported by our findings of' a dose-response relation for each of these. Even though Ntr Lee reaffirrrls his view that mssclassificaton of acuve smoking state can expYktn the average risk ofl lung cancer with passtvee smakLng, we welcome his implieation that the effect of passive smoking on ischaemic heart disease is worth further iavestigation. DAVID I HOLE C33MLES R GILLIS ~ansof 5. wund Ciincer Sur.eillance Gmu, Ruc6s11 h.,.p, W; , GlasgoW :.:0oNB Deynmam a ~:Epbem,oloty~. Um.ersirv~ of SsmG,pn~, slic~eae, UnnN. S~utn VICT.DRM H.aKTHOR7.-E I Hok Dl: Gillss CR. Chopn C. Ha,sberne V,11: Pasnve smokaog . and eueLonspu'a,onhea/P,h ta a een<ral ipopt+l+uon m tM.~'e+e . of Smouad. Br.ued j.I9r9::99:.I3-7. (1: .iugust., `: Thiscorrespondenceisnowclosed;-En,BbfT. TABLE tt-Rkfative ruks found in s:.rdV rtmspared =1th:"rnee" refative nskrfeir acnvr smokers and "oburved"'relantt nSRs;tor f.ai3l-Jes>RIO!!r3' .{cu+c smmkers Poss,ve smokm .11cn u:omen, Both scxes "Tvuc'.' 'True..' .-Observed.. relaure Studyre/iuve Stud}•. rdauve nsk ussdang; nsk 6c,lsngr nsk 'Assumt:.g 7°e ei smwl.crs damstr.uiur.c. Tlhc r-lts ior, bounsases combsnN. nacc been udtussed ior,se<using: WnttASs \.\'.: \.. N;,, unef[ .\ ~:an.t \: are oC-d numC<tS 0~I expVXd and une'SptHe,! Suotern. BFiJ t+OLti.\tE 300 13 )rjNL'.aRY 1990 6~0~ 3~3:' 5-0: 1~34 1 ~14 6~0~ 3s93~, 5-0: 1-19. 1 -14 1 ~0 1-37~ 1 ~a 1~09 1~03 5 0 4.15~ 5.0: 1~31 1 13. - 1-JJ. 1-5: 1 ~ I1 1-05 ' 1 ~8 0~92 ~ 1 ~11 11-02~ :.-0. 1-3^, ~ 7~0 I 1 ~04 3-0 2-S9' 3-0: 2~Dt 1~07~ ?a~0~. 3;33~1 a-0: ?~i9 1~26~ 3 0 `451 3~0: 1~30 1~07. StR,-In her editorial o Professor Eva rilberma rate of deaths from m neural tube defects, in I Paktsnn.' In tne studies deaths were considerea. in this countrx are no screesing programmes, a termrnanon of the pregt5an an affccted'fetus,' so thesr the true incidence of neu women tend to book later fo and this may account for tb neural tube defects to peri tnmester screctting would be smaller proportion of rtsian they may, find tertninauon o able on religious grounds.' the overall incidence of n ascertaining all those affec prenatal screening with u all those found in the pert also tried to determine fact tan[ in explaining: any ra incidence. We reviewed the mat departmentrecords,:neona and necropsyreportsfro end of December 1987 hospital' to ascertain all neonates with,a neural!tu ' dtfe notes were then inspected to detc booking for antcnatal car , if an sound scan was performc ;,and!w uun of pregnancy was off, red. In the Pakistatu'popul an ther tube defects in a tntal! f 3777 b 1000); there were 32 neu I tube d births to white women 1 11 per eongen ' omm orma ts crr . inv a rs e al ,11 Janu on etuse 'erts in rerur 1980-7 Total tu:ural tube defeen Total titnnsIncsdener per. I000 bsrshs Dapcted bv rouwe uh scan Pregtanc. terrtunatcd Pregnancv.mnunt:ed: \otaetecxed bv rouunexan I Scan not arsdablr. L' Not detecsed bv son Booked Yoo lue for sna I Did nos ancnd forscao il malformauons nts on the excess ons, parncularn' f a5others born in to onlc pennatal ural¢uba defects ted by prenatal bmen: ma}• opt (or hen found to have es may not reflect ube defects. Asian heir antenatal care,' igh contribuuon of al mortalirv: second aila bie to a rdatively, men. Furthermore, regnancy,unaceept- e have investigated tube defects by fetuses detected tiy o¢nohy, as well as period. We have at may be impor- ifferenees in the ultrasonogtaphv labour repstens„ 1980 until the district general stillbirths, and The maternal' ne the date af' whenlan ultra- ther a termina- were 11 neural sf291 per ects in 28 834 10a0) (rtable): Routine examination duced'only in 1984 and manyy of the women in tnc dence of neunl' tu populauon was ngninca tlv hiehcrthan that i ~htte population (p=0 013, Fisher's exact taiied test:, reuuve ns 2 i:. 95¢6 conttd interval I`191o 5•3;):~ e woman in each.er taoked'too late ior rouu c prc,=al screemng, one Pakistani woman fa d to attend for the scAn These numbers are smal but n ts o( natc chat t mean gestation at wntca Jiesc women booked w 1+2weeksin.the Paki ani group as compar wtth:14- 3'w'eeks+n the ..-~tte group Six of the 1!1 Astan abus wsth neural tu defects were barnto wom n with aconsangutneous marnage. \C'e have shown tha[ ere is a real mereased. Inctdence ofneunf tube etects:nehe P.ilztstampopulatton, with late booktngand reluecance too terminate an, affected q.rernancs" cantrtbutsn:mtnimalh•: to the tncrcasedInc:den<e inundi . In pertnatal deaths: Changesiin customs 3re ltincui.l to encourace but mac wellloccur, ser7ntaneousic as~~ th ultnsound' was intro- encrwas not a«d bie ta uded in tfus stud The defects in the Pak stani ithe two nce iupp nd 121 Endpoms Studv 6md,mg, !h ~tised pnl<snn 4 03 Phlvcna pntcgrn +23 D spssoea 1 65 Hvpenccrcuon 2 95 .lncsna ' 13 tt ,uraSnonnalsnane/ectroeardaogtnm. 1 57 ~ll causn oI dQ n 1 55 lxhaer.uc nean d~saase 1 36 ~4nC:aI1Ce' j 19 .{ll causea ui dda.'s r<u teu to smuwez 90