Returns the inverse of the onetailed probability of the chisquared distribution. If probability = CHIDIST(x,...), then CHIINV(probability,...) = x. Use this function to compare observed results with expected ones to decide whether your original hypothesis is valid.
Syntax
CHIINV(probability,degrees_freedom)
Probability is a probability associated with the chisquared distribution.
Degrees_freedom is the number of degrees of freedom.
Remarks

If either argument is nonnumeric, CHIINV returns the #VALUE! error value.

If probability < 0 or probability > 1, CHIINV returns the #NUM! error value.

If degrees_freedom is not an integer, it is truncated.

If degrees_freedom < 1 or degrees_freedom ≥ 10^10, CHIINV returns the #NUM! error value.
CHIINV uses an iterative technique for calculating the function. Given a probability value, CHIINV iterates until the result is accurate to within ± 3x10^7. If CHIINV does not converge after 100 iterations, the function returns the #N/A error value.
Example
Probability 
Degrees_freedom 
Formula 
Description (Result) 
0.05 
10 
=CHIINV([Probability],[Degrees_freedom]) 
Inverse of the onetailed probability of the chisquared distribution (18.30703) 