CHIDIST

What it does

CHIDIST(x, deg_freedom)

CHIDIST(x, deg_freedom) returns the chi-square probability to the right of x. CHIDIST is the compatibility name for the right tail.

Try it

=CHIDIST(2,2) is about 0.367879441171442. At two degrees of freedom the right tail is exp(-x/2). The tail at x=0 is one.

Quirks

Degrees of freedom are truncated. Negative x and an admitted degree count below one or above 10^10 give #NUM! in the current shared family. Nonnumeric arguments give #VALUE!. A right tail is not a density.

How to implement

For df=1 the current route calls ERFC.PRECISE at sqrt(x/2), preserving divide-then-square-root order. For df=2 it calls the measured public EXP at -(x/2). Otherwise evaluate the regularized upper gamma ratio Q(df/2,x/2) directly. Avoid obtaining a tiny tail by subtracting a near-one CDF. The worked df=2 value follows from x/2=1. The numerical outline is an implementation route, not an identification of every Excel approximation or return-rounding stage. The current problem-functions report retains unresolved distribution mismatches.

Rust code

22 chidist "CHIDIST" (2, 2) [V "x", V "deg_freedom"] |s, a, sv| {
    let [x, df] = nums::<2>(s, a)?;
    out(s, cft::chisq_dist_rt_kernel(x, df))
}

No implementation has been published with a Handbook test suite for this function.

Version differences

The recorded environments below distinguish a reading from a same-input comparison. Counts describe captured inputs in each environment; overlapping captures and comparisons are not totals of unique worksheet calls.

• XC-CHIDIST-001: build 19929 versus build 20131; 10 same-input pairs, 0 differing answers. Compared axes: excel-build. Numeric answers were compared by their recorded binary64 bits; other answers by typed value.

Agreement on these compared inputs does not establish agreement on unmeasured inputs.

Recorded environments and comparison details.

Alternatives

CHISQ.INV.RT inverts the corresponding probability. CHISQ.TEST computes a test probability from observed and expected counts.

Named variants

No named variants are recorded yet.