How do you calculate grouped data percentile?

The formula for calculating median on grouped data is

L + I *(50% * N-F)/f

L - lower limit of the median group

I - group interval

N - Number of frequency (total sum of frequencies in each group)

F - cumulative freqency for the group before the median group

f - frequency of the median group

Since median is just the same as the 2nd quartile, we use 0.5 in place of 50% in the formula. We can tweak the formula a little bit to calculate any percentile.

For example, if you want to calculate 35th percentile, change the formula to

L + I *(35% * N-F)/f

which is

L + I *(0.35 * N-F)/f.

Please note that L,I,F & f should reflect that of the group where the percentile falls. You can find this by these steps:

1) Calculate N * 0.35. Lets say N=50 then 50* 0.35 = 17.5.

2) Using cumulative frequency, identify the group where 17.5 falls.

3) Use L,I,F & f for that particular group in the formula L + I *(0.35 * N-F)/f