A manufacturer claims that a model of mobile phone has a battery life of 1020 minutes (i.e. 17 hours). In order to test this claim, 20 mobile phones of that model were fully charged and then used to perform a range of tasks. One task involved browsing the web continuously until the phones shut down. The length of time for each phone to shut down was recorded in minutes.

The results are summarised in Table 2.

Table 2 Summary statistics for the battery life (in minutes) of a model of mobile phone Task Sample size Mean Standard deviation Web browsing 20 986 49