18 . The width of photo frames , in centimetres , sold in a bazaar is studied . 11 photo frames are randomly selected and the corresponding widths are measured . It is given that the sample mean is 38 cm and the width of the 99 % confidence interval for the population mean is 13.8 cm .
( a ) Explain why the 90 % confidence interval for the population mean is a subset of the 99 % confidence interval for the population mean .
[ 1 ]
( b ) Write down the 99 % confidence interval for the population mean . [ 1 ]
Let
2
� be the known population variance .
( c ) Find
2
� . [ 4 ]
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