Considering the following data:

A

B

C

Response

$-1$

$-1$

$-1$

$60$

$1$

$-1$

$-1$

$72$

$-1$

$1$

$-1$

$54$

$1$

$1$

$-1$

$68$

$-1$

$-1$

$1$

$52$

$1$

$-1$

$1$

$83$

$-1$

$1$

$1$

$45$

$1$

$1$

$1$

$60$

The main effect value for factor C is equal to: