995
LOG-LINEAR MODEL VAR!ABLES
discouragement), the mean was 4-0 (0’1). Nevertheless, perceived discrimination during teaching sessions was a mean of 3-0 (02) (where 1= no discrimination and 5 = blatant discrimination). There uniform agreement that structured career advice, realistic and more role models would be welcomed. Invited comments showed that medical staff in the obstetrics and gynaecology and orthopaedics/accident and emergency departments were especially lacking in encouragement. The organisation Women in Surgical Training looks after the needs of women in the training grades of the surgical specialties, and is backed by both government and the Royal College of Surgeons. Surely women students who have recognised the considerable difficulties of a surgical career and yet wish to embark on such a venture, deserve guidance and aoorooriate advice? was
part-time training schemes,
John Radcliffe
Hospital, Headington, Oxford OX3 9DU, UK
P.
JANE CLARKE
Reliability of population and prevalence estimates SIR,-Although methods used by ecologists can be applied to estimation of prevalence of non-communicable diseases as Professor Laporte and his colleagues suggest (Feb 22, p 494), we agree with Dr Hook and Dr Regal’s point (March 21, p 742) that the application of the two-sample, capture-recapture formula can result in underestimates or overestimates if the samples are not randomly drawn and independent of each other. However, if three (or more) samples are available, it is possible to take account of interactions between samples by use of log-linear analysis and to incorporate dependencies when estimating the size of the unknown population. This method has lately been used in estimation of the number of drug injectors in Glasgowl-a group of particular concern because of their role in transmitting HIV through sharing of injecting equipment. Non-attributable identifiers (sex, date of birth, initials, first-part postcode) were obtained from three sources reporting contact with injectors during 1989 (518 cases from the Scottish HIV reporting scheme; 968 from treatment agencies; 409 from Strathclyde police). Overlaps were identified by the criteria of identical gender, date of birth, and surname initial (figure). Of cases selected on these criteria over 95% also had the same forename initial and 72% the same first-part postcode. Log-linear analysis was then used to model the interactions between samples. In the table, model 2c incorporating positive dependency between the HIV-test and treatment samples fits the data very well. The number of unknown injectors was then estimated from formulae by Bishop et aF which are adjusted for the particular form of non-independence found between samples of injectors (other formulae are provided for other types of non-independence). From the data for the 1738 individuals represented in the figure, we
Reported
cases of
drug injectors, Glasgow,
1989.
DF= degrees of freedom.
estimated that the total population of injectors in Glasgow was 9400 (SD 1230). A related procedure that involves distinguishing new fieldwork contacts from repesL contacts and recording the decreasing ratio of new to repeat contacts has lately been used to estimate the prevalence of drug-injecting and non-drug injecting female prostitutes in Glasgow.3 Although there are considerable difficulties in estimating the prevalence of covert, illegal, and stigmatised activities, log-linear modelling provides a set of techniques that enable more accurate estimates than have hitherto been available. Cormack’has described both the advantages and drawbacks of log-linear modelling, together with applications that are easily implemented with the GLIM statistical software package.s Communicable Diseases (Scotland) Unit, Ruchill Hospital, Glasgow G20 9NB, UK
MARTIN FRISCHER
Public Health Research Unit,
Glasgow 1 Frischer M. Estimated
ALASTAIR LEYLAND prevalence of injecting drug use m Glasgow. Br J Addiction
1992; 87: 235-43. 2. Bishop Y, Fienberg S, Holland P. Discrete multivariate analysis: theory and practice.
Cambridge, Mass: MIT Press, 1975. 3. Bloor M, Leyland A, Barnard M, McKeganey N. Estimating hidden population: a new of calculating the prevalence of drug-injecting and non-infecting female street prostitution Br J Addiction 1991; 86: 1477-84. 4. Cormack RM. Log-linear models for capture-recapture. Biometrics 1989; 45: 395-413. 5. Payne CD. The GLIM (Generalized Linear Model) system-release. Oxford: Numerical Algorithms, 1986: 77.
Drug aid for the former Soviet Union SIR,-I was distressed to read Leo Offerhaus’ indictment (March 7, p 607) of the humanitarian assistance efforts directed towards the
republics of the former Soviet Union by private organisations and governments of western nations. As a physician volunteer for one of those organisations that is deeply committed to providing pharmaceuticals and medical supplies to the Commonwealth of Independent States, and the newly independent Baltic and eastern European countries, I find Offerhaus’ criticism both unfortunate and uninformed. Through President Bush’s initiative to provide humanitarian assistance and under the auspices and direction of PROJECT HOPE, over US$36 million in appropriate and time-dated pharmaceuticals and supplies have been collected, triaged, and distributed to hospitals and other medical facilities throughout the CIS, Baltic, and eastern European countries over the past 14 months. This effort needed the voluntary services of a large number of individuals, private organisations, pharmaceutical and medical supply companies, and governmental agencies in the USA, and was matched by efforts of other western countries throughout the world. To ensure the appropriateness of donated pharmaceuticals and medical supplies, PROJECT HOPE medical and pharmacy staff evaluate, accept, and allocate each donated product, on the basis of its shelf life and value to the accepting medical institution. This product is allocated to an institution only after an initial on-site assessment of that facility by HOPE professional staff to ensure that the product can be stored and used in a safe and effective way. After allocation and delivery, follow-up visits are conducted to confirm the value and efficacy of the product delivered. This is a very labour-intensive operation, especially in view of the fact that much of the medical expertise has been supplied by physician volunteers willing to donate their time to this important cause. Nevertheless, Offerhaus is probably correct in stating that some organisations and companies have donated and distributed product that is inappropriate or trivial. It is my first hand impression that
LOG-LINEAR MODEL VAR!ABLES
discouragement), the mean was 4-0 (0’1). Nevertheless, perceived discrimination during teaching sessions was a mean of 3-0 (02) (where 1= no discrimination and 5 = blatant discrimination). There uniform agreement that structured career advice, realistic and more role models would be welcomed. Invited comments showed that medical staff in the obstetrics and gynaecology and orthopaedics/accident and emergency departments were especially lacking in encouragement. The organisation Women in Surgical Training looks after the needs of women in the training grades of the surgical specialties, and is backed by both government and the Royal College of Surgeons. Surely women students who have recognised the considerable difficulties of a surgical career and yet wish to embark on such a venture, deserve guidance and aoorooriate advice? was
part-time training schemes,
John Radcliffe
Hospital, Headington, Oxford OX3 9DU, UK
P.
JANE CLARKE
Reliability of population and prevalence estimates SIR,-Although methods used by ecologists can be applied to estimation of prevalence of non-communicable diseases as Professor Laporte and his colleagues suggest (Feb 22, p 494), we agree with Dr Hook and Dr Regal’s point (March 21, p 742) that the application of the two-sample, capture-recapture formula can result in underestimates or overestimates if the samples are not randomly drawn and independent of each other. However, if three (or more) samples are available, it is possible to take account of interactions between samples by use of log-linear analysis and to incorporate dependencies when estimating the size of the unknown population. This method has lately been used in estimation of the number of drug injectors in Glasgowl-a group of particular concern because of their role in transmitting HIV through sharing of injecting equipment. Non-attributable identifiers (sex, date of birth, initials, first-part postcode) were obtained from three sources reporting contact with injectors during 1989 (518 cases from the Scottish HIV reporting scheme; 968 from treatment agencies; 409 from Strathclyde police). Overlaps were identified by the criteria of identical gender, date of birth, and surname initial (figure). Of cases selected on these criteria over 95% also had the same forename initial and 72% the same first-part postcode. Log-linear analysis was then used to model the interactions between samples. In the table, model 2c incorporating positive dependency between the HIV-test and treatment samples fits the data very well. The number of unknown injectors was then estimated from formulae by Bishop et aF which are adjusted for the particular form of non-independence found between samples of injectors (other formulae are provided for other types of non-independence). From the data for the 1738 individuals represented in the figure, we
Reported
cases of
drug injectors, Glasgow,
1989.
DF= degrees of freedom.
estimated that the total population of injectors in Glasgow was 9400 (SD 1230). A related procedure that involves distinguishing new fieldwork contacts from repesL contacts and recording the decreasing ratio of new to repeat contacts has lately been used to estimate the prevalence of drug-injecting and non-drug injecting female prostitutes in Glasgow.3 Although there are considerable difficulties in estimating the prevalence of covert, illegal, and stigmatised activities, log-linear modelling provides a set of techniques that enable more accurate estimates than have hitherto been available. Cormack’has described both the advantages and drawbacks of log-linear modelling, together with applications that are easily implemented with the GLIM statistical software package.s Communicable Diseases (Scotland) Unit, Ruchill Hospital, Glasgow G20 9NB, UK
MARTIN FRISCHER
Public Health Research Unit,
Glasgow 1 Frischer M. Estimated
ALASTAIR LEYLAND prevalence of injecting drug use m Glasgow. Br J Addiction
1992; 87: 235-43. 2. Bishop Y, Fienberg S, Holland P. Discrete multivariate analysis: theory and practice.
Cambridge, Mass: MIT Press, 1975. 3. Bloor M, Leyland A, Barnard M, McKeganey N. Estimating hidden population: a new of calculating the prevalence of drug-injecting and non-infecting female street prostitution Br J Addiction 1991; 86: 1477-84. 4. Cormack RM. Log-linear models for capture-recapture. Biometrics 1989; 45: 395-413. 5. Payne CD. The GLIM (Generalized Linear Model) system-release. Oxford: Numerical Algorithms, 1986: 77.
Drug aid for the former Soviet Union SIR,-I was distressed to read Leo Offerhaus’ indictment (March 7, p 607) of the humanitarian assistance efforts directed towards the
republics of the former Soviet Union by private organisations and governments of western nations. As a physician volunteer for one of those organisations that is deeply committed to providing pharmaceuticals and medical supplies to the Commonwealth of Independent States, and the newly independent Baltic and eastern European countries, I find Offerhaus’ criticism both unfortunate and uninformed. Through President Bush’s initiative to provide humanitarian assistance and under the auspices and direction of PROJECT HOPE, over US$36 million in appropriate and time-dated pharmaceuticals and supplies have been collected, triaged, and distributed to hospitals and other medical facilities throughout the CIS, Baltic, and eastern European countries over the past 14 months. This effort needed the voluntary services of a large number of individuals, private organisations, pharmaceutical and medical supply companies, and governmental agencies in the USA, and was matched by efforts of other western countries throughout the world. To ensure the appropriateness of donated pharmaceuticals and medical supplies, PROJECT HOPE medical and pharmacy staff evaluate, accept, and allocate each donated product, on the basis of its shelf life and value to the accepting medical institution. This product is allocated to an institution only after an initial on-site assessment of that facility by HOPE professional staff to ensure that the product can be stored and used in a safe and effective way. After allocation and delivery, follow-up visits are conducted to confirm the value and efficacy of the product delivered. This is a very labour-intensive operation, especially in view of the fact that much of the medical expertise has been supplied by physician volunteers willing to donate their time to this important cause. Nevertheless, Offerhaus is probably correct in stating that some organisations and companies have donated and distributed product that is inappropriate or trivial. It is my first hand impression that
