How would I use interpolation to find an estimate of the median and quartiles of a frequency distribution?

I've always struggled with statistics.(Crying)

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- February 1st 2010, 12:59 PMQuackyInterpolation
How would I use interpolation to find an estimate of the median and quartiles of a frequency distribution?

I've always struggled with statistics.(Crying) - February 8th 2010, 12:05 PMQuacky
I'm sort of alright with the median, it's the quartiles which I get wrong.(Worried)

- February 9th 2010, 02:07 AMHallsofIvy
The first quartile is where 25% of the data is less than or equal to the number and the third quartile is where 75% of the data is less than or equal to the number. I'm not sure what you mean by "interpolation". If you are saying that, say, the 25% point lies between two data points, taking halfway between is perfectly valid.

- February 9th 2010, 08:00 AMQuackyInterpolation - just a weird word?
I copied that word from an example question. Essentially, the question gave a distribution such as the one below, and asked me to 'using interpolation, estimate the values of the quartiles' I know that the lower and upper quartile both have seperate formulae, I just can't remember them and they don't seem to be displayed upon the internet.

Interval Frequency

00<x<05 6

06<x<10 10

11<x<20 12

21<x<35 4

35<x<40 2

Which is an example I've made up off of the top of my head, how would I estimate the quartiles without drawing a cumulative frequency graph or the like?