what is the formula in getting the percentile of grouped data?

Question

please be specific and determine what are the meaning of the symbols

Answer 1

You need to be a little more specific in your question but I'll try...
To get the percentile you need to normalise the data. that means you need to convert it from whatever form it's in, to a data set with a mean of 0 and a standard deviation of 1.
The mean is simply the average, that is, you add up all the values and divide by the number of values that you have. For grouped data that would be
sum of (number in each group)x(value for each group) / total number in sample.
The standard deviation is a little harder, that's a measure of how spread out the data is. I'm not going to try and write out the formula here, with limitations of this YA text editor, but check out this link:
http://en.wikipedia.org/wiki/Standard_deviation#See_also
Ok, now that you have the mean and the standard deviation, the next step is to convert your data into normalized data also known as "z scores". To do this, take each value, subtract the mean, and divide by the standard deviation. You're almost done...
Finally, look in the body of the normal table and find the percentage corresponding to the percentile you're interested in. Remember the normal table goes from 0 to .5, because it's symetric, so if you're looking for, say, the 90th percentile you'd look for a number close to .45 in the body of the table. Then look at the margins to find the corresponding z score. Finally, convert this z score back into raw scores by multiplying by the standard devaition then adding the mean.
Did I say earlier that some questions lend themselves a lot better to face to face tutoring than online explanations? This is one of them. By now your head is probably spinning and you wish someone had just given you a few lines of alphabet soup. But a good tutor, sitting beside you and showing you step by step, could make all tihs make sense and help you understand.

Answer 2

Percentile Formula For Grouped Data

Answer 3

Assume you have a group of data:
a1, a2, a3, ..., an

the percentile of ai = [ai/(a1+a2+a3+...+an)][100%]

"i" can be any of the integers from 1 to n.