The mothers who experienced normal births were matched with the other mothers for age and reproductive history.
Swedish general population registers were used to identify a cohort of children who were admitted to hospital due to respiratory infection in their first year after birth and they were matched with a cohort of children not admitted to hospital for infection in their first year.
Control births were matched 5 1 with replacement by maternal age (one year either way), parity (nulliparous or parous), body mass index in early pregnancy (18.5-24.9, 25-29.9, 30-34.9, 35-39.9, ≥40, or missing), smoking status in early pregnancy (non-smoker, 1-9 or ≥10 cigarettes daily, or missing), educational level (≤9, 10-12, or >12 years of schooling, or missing), and delivery year (1992-2009).
Cases were matched with controls by birth hospital and gestational age.
Oral cleft, CHD, and NTD cases in the Colorado Responds to Children with Special Needs (CRCSN) birth registry, obtained from hospital records, the Newborn Genetics Screening Program, the Newborn Hearing Screening Program, laboratories, physicians, and genetic, developmental, and other specialty clinics (CRCSN 2011) were matched with Colorado live birth certificates.
Controls were matched with cases by age, birth year, and gender.
Three useful precepts for judging negative results are The power of a study may be inadequate to reveal an important effect and it is true that because the controls in the NRCT were matched with cases from the same birth registers they are partially matched with respect to our principal explanatory variable, namely proximity to the nearest NPP.
Control subjects were matched with cancer subjects 1 1 by date of birth, sex, and smoking history.
Controls (n=7954) were matched with each case on general practice site, sex and year of birth.
The vast majority of surveys found that over 90% of short-spaced births could be matched with a regular-spaced birth.
