Why is Back-End Plus Front-End Not Equal to the Total Page Load Time?

Document created by DPM Admin Employee on Jul 13, 2017
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Have you ever looked at your front-end, back-end, and total page load time medians, and wonder why they do not add up?

Example 1

  • Total = 1.17s
  • Back-End (BE) = 625ms
  • Front-End (FE) = 1.74s.

As a result, the FE + BE > Total.

The explanation is basic statistics. Medians cannot be compared across different datasets. In this case, the Total, FE, and BE timers are separate datasets, and the median for each is the middle point in each set independent of the other sets.

Example 2

Assume a case with 5 beacons with the following times:

 

 BEFETotal 
1300ms900ms1200ms 
2250ms*1000ms1250ms 
3*275ms800ms1075ms 
4150ms1050ms*1200ms 
5350ms1100ms1450ms 
 
Median275ms1000ms1200ms(BE + FE = 1275ms)
85pctl300ms1050ms1250ms(BE + FE = 1350ms)

 

You have the following result:

  • For each beacon, the sum of BE and FE always add up to total.
  • For each timer, the middle element (median) across all beacons ends up being on a different beacon for each one of them.
  • For each timer, the 85th percentile (pctl) across all beacons ends up being on a different beacon for each one of them.

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