To investigate how infants first comprehend and produce words, six British children were observed in their homes every 2 weeks from age 6 to 18 months [13]. Among the data collected were the age (in days) at which each child first understood spoken English and first spoke a word of English, shown in Table 6.5. Assuming these six children to be a random sample of all British children, determine a 95 percent confidence interval for the average difference (in days) between when a child first understands and speaks English. Does your estimate seem fairly precise? Why is it not more precise? Why might this be a biased sample?

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Understood Age (days) Spoke Difference Andrew 253 368 115 Ben 238 252 14 George 257 408 151 Katherine 220 327 107 Katy 249 268 19 Sebastian 254 326 72