1. What effect does the size of the sample have on the width of the confidence interval?
The larger the sample size the smaller the width of the confidence interval. With a large sample size the mean of the sample is more likely to be nearer to the mean of the population.
2. When carrying out an opinion poll, what value is used for p when working out the sample size for the required degree of accuracy?
A value of 0.5 is used for p. This is because in the formula p(1 − p) the values for 0.5, 0.4 and 0.3 are all very similar anyway and it is best to use the larger value of p(1 − p).
