Maths - Grouped Frequency Tables

[Jules001]

sorry for putting this in this category, but was unsure where else it could go and as it is puzzling me, this category fits!

my son has got some coursework to do on this subject - I have managed to help him as much as I can, but am a bit confused on one part - can anybody please help me along a bit.

He has got a grouped frequency table and needs to find out the modal class (fine I've sorted that) he then needs to estimate the mean ( again fine I've helped him with this) but he also needs to estimate the median which is where we are having trouble. The book states to put all the data in order to find the answer, but we are still confused as to which data they are referring to. We have got the two rows given to us and have added a 3rd row (mid interval value) and a 4th row (frequency times mid interval value) but don't know which rows' data we should be putting in order - hope this makes sense to someone out there and that I have explained it well enough, because it confuses the hell out of me despite me sitting my GCSE in Maths a few years ago

[Jabba]

It's the original data you need to order. Here's an example. Suppose we have counted the number of cars that pass a certain point in an hour on 30 days. We get

1 - 5  7 times

6 - 10 11 times

11 - 15 9 times

16 - 20 3 times

Since there are 30 observations, we need to look at the 15th and 16th to get the median. The first 7 are in the 1 - 5 group; numbers 8 to 18 (7 + 11) are in the 6 - 10 group, so that's where the median is. There are different ways of estimating the median once you've found the right group: some people take the midpoint of that group and some people use linear interpolation.

[Jules001]

Thanks Jabba - you explained it clearly so even I could understand it - not sure about the linear interpolation though :(