90th percentile Response Time is defined by many definitions but it can be easily understood by:
"The 90th percentile tells you the value for which 90% of the data points are smaller and 10% are bigger."

90% RT is the one factor we should always look in once the Analysis report gets generated
To calculate the 90% RT 

1. Sort the transaction RT by their values
2. Remove the top 10% instances
3. The higher value left is the 9oth percentile RT.

For e.g. Consider we have a script with transaction name "T01_Performance_Testing" and there are 10 instances of this transaction, i.e. we ran this transaction for 10 times.
Values of transaction's 10 instances are 

1. Sort them by their values

2. Remove top 10% values, i.e. here 9Sec

3. 8 Sec is the 90th percentile RT.

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