Have you ever looked at your front-end, back-end, and total page load time medians, and wonder why they do not add up?
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.
Assume a case with 5 beacons with the following times:
|Median||275ms||1000ms||1200ms||(BE + FE = 1275ms)|
|85pctl||300ms||1050ms||1250ms||(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.