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Letter to the Editor: About lost data in cohort studies

Published online by Cambridge University Press:  10 June 2016

R. Quiroz-Portella ,
C. Valer-Montoya  and
G. F. Alvarado
R. Quiroz-Portella*
Affiliation:
School of Medicine, Universidad Peruana de Ciencias Aplicadas, Lima, Perú
C. Valer-Montoya
Affiliation:
School of Medicine, Universidad Peruana de Ciencias Aplicadas, Lima, Perú
G. F. Alvarado
Affiliation:
School of Medicine, Universidad Peruana de Ciencias Aplicadas, Lima, Perú
*
Author for correspondence: R. Quiroz-Portella, Avenida Alameda San Marcos, Cuadra 2, Lima 23, Perú. (Email: [email protected])
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Abstract

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Type
Correspondence
Information
Psychological Medicine , Volume 46 , Issue 11 , August 2016 , pp. 2448
Copyright
Copyright © Cambridge University Press 2016 

Clemmensen et al. (Reference Clemmensen, van, Drukker, Munkholm, Rimvall, Vaever, Rask, Bartles-Velthius, Skovgaard and Jeppesen2016) address a very important and interesting topic on how hyper-theory-of-mind (HToM) correlates with psychotic experiences. Their findings correlate with previous research (Clemmensen et al. Reference Clemmensen, van Os, Skovgaard, Vaever, Blijd-Hoogewys, Bartles-Velthius and Jeppesen2014). This research provides new information upon HToM, is a prospective design, has a large sample, and the instruments are validated.

The Copenhagen County Child Cohort 2000 includes 6090 children which is a representative sample of the children born in Denmark that year (Olsen et al. Reference Olsen, Skovgaard, Weile and Jorgensen2007). One important limitation of the present research is the high loss to follow-up (LFU). Only 1630 subjects were included on the analysis out of 2354 that completed the Development and Well-being Assessment. LFU is a type of selection bias; authors have found that when LFU is large, an impact on results is observed (Kristman et al. Reference Kristman, Manno and Coté2004). The latter is especially important when characteristics of subjects not included in the study differ from the final sample. Jeppesen et al. (Reference Jeppesen, Clemmensen, Munkholm, Rimvall, Rask, Jorgensen, Larsen, Petersen, van Os and Skovgaard2015) showed that there are significant differences among the two groups on the variables ‘education of mother’, ‘maternal age’ and other variables (p < 0.05).

Moreover, in the study by Clemmensen et al. (Reference Clemmensen, van, Drukker, Munkholm, Rimvall, Vaever, Rask, Bartles-Velthius, Skovgaard and Jeppesen2016), the χ2 test was used for the bivariate analysis of categorical variables and the association between ‘infancy contact problems’ and HToM does not meet the assumptions for the uncorrected χ2 test; the Fisher exact test is a better option in this case (Rosner, Reference Rosner2015). On the other hand, showing unadjusted (as well as adjusted) odds ratios in Table 4 would help a better understanding of the association between the exposure and the outcome.

Acknowledgements

This research received no specific grant from any funding agency, commercial or non-for-profit sector.

Declaration of Interest

None.

References

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