The 90th percentile is the value for which 90% of the data points are smaller. The 90th percentile is a measure of statically distribution, not unlike the median. The median is the middle value. The median is the value for which 50% of the values were bigger, and 50% smaller. The 90th percentile tells you the value for which 90% of the data points are smaller and 10% are bigger.

Statistically, to calculate the 90th percentile value:
1. Sort the transaction instances by their value.
2. Remove the top 10% instances.
3. The highest value left is the 90th percentile.

Example:
There are ten instances of transaction ''t1'' with the values 1,3,2,4,5,20,7,8,9,6 (in sec).
1. Sort by value -- 1,2,3,4,5,6,7,8,9,20.
2. Remove top 10 % -- remove the value ''20.''
3. The highest value left is the 90th percentile -- 9 is the 90th percentile value.
The 90th percentile value answers the question, ''What percentage of my transactions have a response time less than or equal to the 90th percentile value?'' Given the above information, here is how LoadRunner calculates the 90th percentile.
Each value is counted in a range of values. For example, 5 can be counted in a range of 4.95 to 5.05, 7.2 in a range of 7.15 to 7.25. The 90% is taken from the range of values that the number of transaction in it and before it is >= ( 0.9 * Number of Values).