This study investigated whether babies take longer to learn to crawl in cold months when they are often bundled in clothes that restrict their movement, than in warmer months. The study sought an association between babies' first crawling age and the average temperature during the month they first try to crawl (about 6 months after birth). Parents brought their babies into the University of Denver Infant Study Center between 1988-1991 for the study. The parents reported the birth month and age at which their child was first able to creep or crawl a distance of four feet in one minute. Data were collected on 208 boys and 206 girls (40 pairs of which were twins).
Correlation and regression can be used to examine the relationship between the average crawling age and the average temperature.
The study has some problems:
1. The babies are not all independent because there are twins in the study.
2. The normality assumption is in jeopardy since outliers can only occur as higher ages of first crawling.
3. The study was conducted on self-selected volunteers, who may be different from the population.