Understanding a pdf graph and why you'd use it

[kerrymaid]

Hi,

My question is that I don’t fully understand how to “read” a pdf graph and would appreciate if someone could explain it to me and why you’d bother using it.

I understand in a CDF for example you could draw a line from the 80% value until it hits the curve and then drop a perpendicular to the x axis (lets say this reaches a value of 6) so that you could definitively say “80% of the values are resolved in 6 or less units”.

But I don’t understand how to “read” the pdf. I’ve read about it and know everything under the curve should add to 1 etc and it takes a bell shape but is the purpose to say well the mean (central value on x axis) will be experienced with 30% frequency or something (by drawing a line from central mean to top of curve and then left over to the y axis?

Thanks in advance

 

[tkhunny]

There are infinitely many pdfs. They do not all take a bell shape.

In some respects, the same information is contained in the two graphic forms. However, it is much easier to detect symmetry and skewness in the pdf form. On the other hand, anyone good at reading maps with isobars has no trouble envisioning one from the other.

It is also quite likely that one algebraic form is far simpler. I dare you to express the Standard Normal Curve in a simple algebraic expressions, for example.

You should have both tools in your arsenal.