Answer: effect or consequence
Explanation:There are three part that makes up an assertive message it's the behaviour,feeling and the effect.
Teh behaviour is explained that William want to go and visit over his friend probably because he wants to relax and his father states his own feelings towards this.
The father thought doesn't state what will be the effect for William if he goes over to his friend which is will be probably more influential in changing his mind about going over to his friend .
The lack of the consequences on the message may make the receiver not see why his behaviour need to change or why he may need to rethink it.