# deviance

#### antonitsin

##### New Member
can anybody please tell me why we need to multiply difference of likelyhoods by "-2"?
Where this 2 came from, why not 3?

#### Masteras

##### TS Contributor
Because in the Taylor series expansion of the likelihood ratio test statistic the second term with the second derivative has 2 in the denominator. So you need to multiply by 2 to vanish that 2 in the denominator. As for the fisrt term in the Taylor series it has a denominator 1 but usually is zero or vanishes later.

#### antonitsin

##### New Member
Thanks, that make sense, but what about all other terms? do we assume they be negligible?

Thanks again

#### Masteras

##### TS Contributor
yes, we stop untill the seocnd derivative in the taylor series.