Misclassification bias

Non-differential misclassification

Errors in the measurement of variables are unavoidable in studies where exposures are self- reported, particularly when the information is obtained retrospectively. In the present context, the delay between exposure and interview was relatively short, which may have reduced recall difficulties.

Since most of the mothers had the child’s personal health records (over 95% of cases and 98%

of controls) in their hands, gestational age and birth-weight were unlikely to be misclassified. On the other hand, the interviews could not elicit all the characteristics of specific exposures and, given their complexity, it is likely that mothers may not have been able to determine the agent of true interest. For example, mothers might not be aware that supplementation products for pregnancy contained folic acid, or even that the supplementation product contained a product other than folic acid.

A woman’s total plasma folate level is determined by multiple factors. Dietary folate might be high among mothers that did not report folic acid intake around pregnancy. Dietary folate is absorbed differently from supplemental folic acid and as well as naturally occurring folate. In this study, we were not able to assess the dietary folate intake. In the same line, exposure opportunity and the intensity of pesticide exposure depends on many factors (nature, frequency and amount of pesticides used, methods of application, time spent in the exposed place, etc.) that could not be collected in the interviews. These types of exposure misclassification are likely to be randomly distributed among cases and it may tend to minimize the strength of an association, if present.

With regards to congenital malformations, interview-based studies are subjects to non- differential misclassification bias as they use unconfirmed information on birth defects. In our study, two independent reviewers with medical background assessed the maternal responses about congenital malformations. The reviewers were blinded to the case-controls status of children.

Paternal smoking was assessed through the maternal report of the father’s tobacco consumption. This could introduce a bias if the provided information was not accurate. In this study, the extent of the bias is limited since in the subset used for validation, agreement between maternal and paternal responses was high with regards to both ever smoking and number of cigarettes smoked per day[114]. Furthermore, control parents were also similar to the source population in the same age group in terms of tobacco smoking, when compared to estimates from the 2005 and 2010 French Health Barometer surveys[115]. For example,

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42.5% and 44.3% of fathers reported regular smoking during the ESCALE and ESTELLE studies, respectively. This is similar to 42.6% and 47.7% of men of similar age (26-34 years) that reported regular smoking in 2005 and 2010, respectively.

Recall bias

The main concern about interview-based case-control studies is that the mothers of cases may recall previous exposures differently than the mothers of controls. The ESCALE and ESTELLE studies were designed to reduce the risk of recall bias by the use of computer- assisted standardized interviews conducted by the same trained interviewers, contemporaneously and in identical conditions for cases and controls, although not blind to the case-control status. For example, over-declaration of malformations among neuroblastoma cases in which case mothers are more likely to remember minor defects than control mothers, may introduce differential classification bias in case-control studies. In order to limit this potential bias, we have excluded minor anomalies that are not always truly congenital in origin, sometimes associated with immaturity at birth, and have lesser medical or functional consequences. The criteria for exclusion were based on EUROCAT group

recommendations[100], as the group’s experience showed that the definition, diagnosis and

reporting of minor malformations vary considerably, while major malformations are less liable to differential recall bias. Despite the fact that recall bias cannot be completely ruled out, other studies based on birth records provide support for the validity of our findings[44], [47], [48], [53], [59], [60]. Another concern was the fact that medical diagnosis of some types of malformation may be more frequent among cases. For example, since most of neuroblastoma tumors are located in the abdominal cavity, the use of tomography scan for risk stratification may also lead to the diagnosis of an asymptomatic malformation, such as renal agenesis. Although we cannot exclude this possibility, a specific question was included in the ESTELLE study and none of the cases declared that the malformation had been diagnosed in the context of another pathology.

Pesticide exposure and maternal smoking can also be subject to recall bias, and the direction of the bias is impossible to foresee. Mothers of cases may have been thinking more deeply and have less under-reported pesticide exposures than control mothers. The opposite scenario may also be possible and a true association may be underestimated. Deleterious effects of maternal smoking during pregnancy are well known and potential carcinogenic effects of pesticides raise concern in society. As social desirability is known to influence self-report of

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substance abuse[116] case mothers may under-report these exposures if they try to deny any responsibility for the disease.