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Are the results valid?1. Was there a clearly defined, focused research question? What was the study question?

The aim of this study was to assess the prevalence of excessive crying at the age of 3 months and to test the hypothesis that excessive crying at this age is associated with behavioural problems when children reach the age of 4 years.

(Page 1. Introduction)

2. How was the exposed cohort selected? Was there a well defined selection procedure for inclusion into the cohort? What proportion of eligible subjects was actually included?

A birth cohort was started in 2004 in the city of Pelotas (Southern Brazil) when all live births wereincluded between January 1st and December 31st.Only newborns to mothers living in the urban areaof Pelotas were eligible to the study. Of a total of 4263 mothers complying with this inclusion criterion, 32 (0.8%) refused to participate in the perinatal study.

(Page.1. Method)

3. How was the non-exposed cohort selected? Was this cohort drawn from the same source population as the exposed cohort? Was there a well defined selection procedure for inclusion into the cohort?

The Non-exposed cohort selected drawn from the same source population as the exposed cohort.

A birth cohort was started in 2004 in the city of Pelotas (Southern Brazil) when all live births wereincluded between January 1st and December 31st.Only newborns to mothers living in the urban areaof Pelotas were eligible to the study.

(Page.1. Method)

4. How were the main exposures ascertained? Were the exposures clear, specific and measurable?

Excessive crying was assessed at the 3-month visitby asking the mother whether“compared to babiesof the same age, her baby cried more, less or as thesame”. Infants whose mothers perceived them as crying more than others of the same age were classified as ‘crying babies’.

(page 1-2, Exposure)

Child behaviour was assessed through the application to the mother of the 4–18 year version of the Child Behavior Checklist (CBCL) 25 when children were 4 years of age.

(Page 2. Outcome)

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5. Were only people at risk of the outcome included?

Yes

In the crude analysis crying babies had higher risk of being in the clinical range for CBCL total, internalising and externalising problems than babies who did not cry a lot. (Table.3)

(Page 3.)

6. Was duration of follow-up adequate (i.e. long enough for main outcomes to occur)?

Yes, the subject being followed up at 3, 12, 24, 48 and 72 months of age.

(Page 1. Method)

7. Was follow-up complete? Were efforts made to limit the loss to follow-up?

Follow-up complete. Tetapi pada jurnal tidak dijelaskan usaha apa saja yang dilakukan untuk meminimalkan loss follow-up

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8. What were the primary and secondary outcomes of the study?

Child behaviour was assessed through the application to the mother of the 4–18 year version of the Child Behavior Checklist (CBCL) 25 when children were 4 years of age.

(Page.2. Outcome)

9. Were outcomes measures similarly in exposed and non exposed cohorts?

Yes

Among children with excessive crying at 3 months the proportion in the clinical range for CBCL total, internalising and externalising problems at 4 years of age was 31.2%, 12.9% and 37.5%, respectively. Among non-crying babies CBCL total, internalising and externalising problems were 20.6%, 6.8% and29.6%, respectively. (Table.2)

(Page 2. Result)10. How comparable were the exposed and non-exposed cohorts? Have the authors identified all potentially important confounders? Is there information on how the potential confounders are distributed between the comparison groups? What confounders were adjusted for and was the adjustment adequate? Is residual confounding a concern?

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Any other potential biases in this study?

Potential for selection bias?

Potential for information bias?