At what age do babies learn to crawl? Does it take longer to learn in the winter

Question

At what age do babies learn to crawl? Does it take longer to learn in the winter when babies are often bundled in clothes that restrict their movement? Data were collected from parents who brought their babies into the University of Denver Infant Study Center to participate in one of a number of experiments between 1988 and 1991. Parents reported the age (in weeks) at which their child was first able to creep or crawl a distance of four feet within one minute. The resulting data were grouped by month of birth. The data are for January, May, and September.
Birth Month Average Crawling Age SD n January 29.84 7.08 32 May 28.58 8.07 27 September 33.83 6.93 38

Crawling age is given in weeks. Assume that data are three independent simple random samples, one from each of the three populations (babies born in a particular month), and that the populations of crawling ages have Normal distributions. The overall mean response is A 30.75. B 31.05. C 7.30.

Explanation / Answer

The overall mean response x_bar is Sum(x * n(x))/Sum(n(x)) = [29.84*32 +28.58*27+33.83*38] / (32+27+38)=31.05

Hence, B is the correct answer

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