- Email: [email protected]
The UK Biobank and selection bias
Correspondence
Use of more disaggregated data alone cannot overcome the inherent limitations of the model (eg, primarycare quality being based solely on the blood pressure and cholesterol concentrations of people with cardiovascular disease; hospital safety being based on hospital standardised mortality ratios—a measure that has never been validated). To claim that his model provides “a comprehensive measure of quality” suggests a rather limited view of what constitutes health-care quality. The need for an accurate measure of health-care productivity remains a high priority. This extremely challenging task would be best achieved by those responsible not adopting such a defensive position. I declare that I have no conflicts of interest.
Nick Black [email protected] Department of Health Services Research & Policy, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK
twice that of the general population, for three diagnoses about three times, and so forth” and that “the spurious correlations referred to are not a consequence of any assumptions regarding biological forces, or the direct selection of correlated probabilities, but are the result of merely ordinary compounding of independent probabilities”. On the basis of this fundamental principle and example, it might be hazardous to estimate the comorbidity of diseases from the UK Biobank. Heckman4 discusses sample selection bias as a specification error and presents “a simple consistent estimation method that eliminates the specification error for the case of censored samples”. Perhaps this method should be considered for use in analyses of data in the UK Biobank. I declare that I have no conflicts of interest.
James M Swanson [email protected] University of California—Irvine, Irvine, CA 92671, USA 1
The UK Biobank and selection bias
Wellcome Photo Library
I agree with Rory Collins (March 31, p 1173)1 that the UK Biobank is special, but I would like to comment on one assertion: “Generalisable associations of exposure with disease can be obtained without including representative samples of particular populations.” A sample of 500 000 was recruited with remarkable speed and efficiency,2 but this efficiency was achieved at the expense of response rate (5·5%) and was subject to selection bias. The warning given by Berkson3 should be noted: if selective probabilities operate independently to draw individuals to a sample, these diseases might seem to be associated when they are not. He showed that “the ratio of multiple diagnoses to single diagnoses in the [sample] will always be greater than in the population; for two diagnoses the ratio will be about 110
2
3
4
Collins R. What makes the UK Biobank special? Lancet 2012; 379: 1173–74. Manolio TA, Weis BK, Cowie CC, et al. New models for large prospective studies: is there a better way? Am J Epidemiol 2012; 175: 859–66. Berkson J. Limitations of the application of fourfold table analysis to hospital data. Biometrics Bull 1946; 2: 47–53. Hechtman JJ. Sample selection bias as a specification error. Econometrica 1979; 47: 153–61.
driven by overzealous medical experts, seemed to be that mothers could get away with it once but not twice. The problem, of course, is that we do not understand the natural disease processes that lead to sudden unexpected death in infancy and in that setting false accusations can occur. A current example of the same problem is so-called abusive head injury. An infant collapses and develops hypoxic ischaemic encephalopathy with bilateral retinal haemorrhage and bilateral thin-film subdural haemorrhage.3 Even in the complete absence of any external evidence of trauma, the diagnosis of abusive head injury will follow and the mother or carer is likely to go to prison. Experts will argue that there is no other cause than trauma and all natural disease has been excluded. Ignorance combined with bleeding is a potent force for injustice. Harrington states: “Today, memories of that whole era make many people wince.” Those who are too ready to accuse mothers should bear this in mind—history will not be kind. I have given expert evidence, called by the defence, in criminal cases in which mothers have been accused of causing the death of their infants.
J A Morris [email protected] University Hospitals of Morecambe Bay NHS Trust, Royal Lancaster Infirmary, Lancaster LA1 4RP, UK 1
The fall of the schizophrenogenic mother
2
3
Harrington A. The fall of the schizophrenogenic mother. Lancet 2012; 379: 1292–93. Supreme Court of Judicature, Court of Appeal (Criminal Division). Neutral citation number: (2004) EWCA Crim. 01 Case No: 200201711D3 Regina versus Angela Cannings. Gerber P, Coffman K. Non-accidental head trauma in infants. Childs Nerv Syst 2007; 23: 499–507.
The essay by Anne Harrington (April 7, p 1292)1 is timely because the attitudes that led to the concept of the “schizophrenogenic mother” are still prevalent in medicine and the law. It is too easy, when faced with a child with a condition that we do not understand, to blame the parent or carer. Only a few years ago women were being sent to prison when they suffered the tragedy of a second cot death.2 The attitude of the prosecuting authorities, www.thelancet.com Vol 380 July 14, 2012
Use of more disaggregated data alone cannot overcome the inherent limitations of the model (eg, primarycare quality being based solely on the blood pressure and cholesterol concentrations of people with cardiovascular disease; hospital safety being based on hospital standardised mortality ratios—a measure that has never been validated). To claim that his model provides “a comprehensive measure of quality” suggests a rather limited view of what constitutes health-care quality. The need for an accurate measure of health-care productivity remains a high priority. This extremely challenging task would be best achieved by those responsible not adopting such a defensive position. I declare that I have no conflicts of interest.
Nick Black [email protected] Department of Health Services Research & Policy, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK
twice that of the general population, for three diagnoses about three times, and so forth” and that “the spurious correlations referred to are not a consequence of any assumptions regarding biological forces, or the direct selection of correlated probabilities, but are the result of merely ordinary compounding of independent probabilities”. On the basis of this fundamental principle and example, it might be hazardous to estimate the comorbidity of diseases from the UK Biobank. Heckman4 discusses sample selection bias as a specification error and presents “a simple consistent estimation method that eliminates the specification error for the case of censored samples”. Perhaps this method should be considered for use in analyses of data in the UK Biobank. I declare that I have no conflicts of interest.
James M Swanson [email protected] University of California—Irvine, Irvine, CA 92671, USA 1
The UK Biobank and selection bias
Wellcome Photo Library
I agree with Rory Collins (March 31, p 1173)1 that the UK Biobank is special, but I would like to comment on one assertion: “Generalisable associations of exposure with disease can be obtained without including representative samples of particular populations.” A sample of 500 000 was recruited with remarkable speed and efficiency,2 but this efficiency was achieved at the expense of response rate (5·5%) and was subject to selection bias. The warning given by Berkson3 should be noted: if selective probabilities operate independently to draw individuals to a sample, these diseases might seem to be associated when they are not. He showed that “the ratio of multiple diagnoses to single diagnoses in the [sample] will always be greater than in the population; for two diagnoses the ratio will be about 110
2
3
4
Collins R. What makes the UK Biobank special? Lancet 2012; 379: 1173–74. Manolio TA, Weis BK, Cowie CC, et al. New models for large prospective studies: is there a better way? Am J Epidemiol 2012; 175: 859–66. Berkson J. Limitations of the application of fourfold table analysis to hospital data. Biometrics Bull 1946; 2: 47–53. Hechtman JJ. Sample selection bias as a specification error. Econometrica 1979; 47: 153–61.
driven by overzealous medical experts, seemed to be that mothers could get away with it once but not twice. The problem, of course, is that we do not understand the natural disease processes that lead to sudden unexpected death in infancy and in that setting false accusations can occur. A current example of the same problem is so-called abusive head injury. An infant collapses and develops hypoxic ischaemic encephalopathy with bilateral retinal haemorrhage and bilateral thin-film subdural haemorrhage.3 Even in the complete absence of any external evidence of trauma, the diagnosis of abusive head injury will follow and the mother or carer is likely to go to prison. Experts will argue that there is no other cause than trauma and all natural disease has been excluded. Ignorance combined with bleeding is a potent force for injustice. Harrington states: “Today, memories of that whole era make many people wince.” Those who are too ready to accuse mothers should bear this in mind—history will not be kind. I have given expert evidence, called by the defence, in criminal cases in which mothers have been accused of causing the death of their infants.
J A Morris [email protected] University Hospitals of Morecambe Bay NHS Trust, Royal Lancaster Infirmary, Lancaster LA1 4RP, UK 1
The fall of the schizophrenogenic mother
2
3
Harrington A. The fall of the schizophrenogenic mother. Lancet 2012; 379: 1292–93. Supreme Court of Judicature, Court of Appeal (Criminal Division). Neutral citation number: (2004) EWCA Crim. 01 Case No: 200201711D3 Regina versus Angela Cannings. Gerber P, Coffman K. Non-accidental head trauma in infants. Childs Nerv Syst 2007; 23: 499–507.
The essay by Anne Harrington (April 7, p 1292)1 is timely because the attitudes that led to the concept of the “schizophrenogenic mother” are still prevalent in medicine and the law. It is too easy, when faced with a child with a condition that we do not understand, to blame the parent or carer. Only a few years ago women were being sent to prison when they suffered the tragedy of a second cot death.2 The attitude of the prosecuting authorities, www.thelancet.com Vol 380 July 14, 2012