The Student Room Group
Reply 1
In general (I'm tempted to say "always"), the more details you know about a set of data, the stronger your confidence about any claims you make.
The t distribution curve has a similar shape to the Z curve (both bell-shaped) but is flatter (i.e. has "more" probability at the extremes). Have a look at wikipedia.
What impact do you think that will have on confidence intervals?
Reply 2
sof the t disttttbuion curve is flatter, then the confidence intervals will be wider, but why then would i be using the t distribution for the first one, could i not use the z distribution for both?
Reply 3
masterasg
sof the t disttttbuion curve is flatter, then the confidence intervals will be wider, but why then would i be using the t distribution for the first one, could i not use the z distribution for both?

the first one? both? :s-smilie:
Reply 4
i guess im trying to ask why cant you use the z distribution for both if the variance is known and not known...
Reply 5
Because those are different situations. The theoretical model upon which the normal distribution is based requires the variance of the population to be known, whereas the model the t distribution is built on only requires an estimate of the population variance from a sample of known size.

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