Why round function results different

[Wen Bai]

. di ((87.8-86.8)*95+(90-87.8)*85)/(90-86.8)
88.125

. di round(88.125,0.01)
88.13

. di round(((87.8-86.8)*95+(90-87.8)*85)/(90-86.8),0.01)
88.12

[Andrew Musau]

The first paragraph running

help precision

is

Description

Stata works in binary. Stata stores data in float precision by default. Stata performs all calculations in double precision. Sometimes the combination results in
surprises until you think more carefully about what happened.

Below, note that in binary, the representation of the two values is not the same.

di %16H 88.125
di %16H `=((87.8-86.8)*95+(90-87.8)*85)/(90-86.8)'

Res.:

. di %16H 88.125
4056080000000000

.
. di %16H `=((87.8-86.8)*95+(90-87.8)*85)/(90-86.8)'
405607ffffffffff

[Nick Cox]

Display to a specified number of decimal places is a problem for a display format, not for a numeric function.

Andrew Musau makes the main point concisely.

One of several longer explanations is in my paper on rounding and binning in the Stata Journal. Type say

search binning, sj

to get a link.

[Nick Cox]

Here's the link I had in mind.

https://www.stata-journal.com/articl...article=dm0095