Hi,

I have a little statistics question about the variance to mean (V/M)

value reported by mk-query digest.

let's consider this output :

# Attribute pct total min max avg 95% stddev

median V/M

# ============ === ======= ======= ======= ======= ======= =======

======= ====

# Count 12 160000

# Exec time 11 152s 0 1s 948us 972us 8ms

972us 0.07

(i have added the v/m column from the profile output).

From my understanding :

* variance V(X) is the mean of the squared distances to the mean

* std dev is the squared root of the variance

* variance to mean = the ratio of the variance (σ) squared to the mean

(µ) : σ^2/ μ

* coefficient of dispersion = variance to mean : σ / µ

For the sample above, we have :

stddev = 0.008 = sqrt(variance)

variance = 0.008^2 = 64µs = 0.000064s

coefficient of dispersion = 0.000064 / 0.000948 ~= 0.07

variance to mean ratio = 0.000064^2 / 0.000948 ~= 0.000004

Based on the sample above, i think mk-query-digest reports the

coefficient of dispersion, not the variance to mean.

Am i correct ?

Cheers,

Romain.