Random Sample of Dates then Average Values

Hi,

I am using Excel 2003 - Can anybody help?

I have a spreadsheet (attached) of various works orders with trades, date completed and time taken to complete (duration). As time goes on the numbers of dates will increase up to one year.

For each date, I need to take a random sample of 5 data points (duration) and then get an average from those 5 points. To then be plotted against the date completed on a chart.

It would be useful to be able to separate out the different trades from the overall data but not essential.

I have been able to do this by taking random samples and averaging using excel's built tools but this is long and tedious. Is there a more intelligent way of making it as automated as possible?

Thanks

Hume