Compound subjects (joined by and) takeVerbs. A present tense plural verb does NOT end in "s".

plural

Explanation:

A compound subject in one which two or more than two of the individual noun phrases are then coordinated to form a single and a longer noun phrase. In other words, a compound subject contains more than one subject or predicate.

The usage of a compound subject is somewhat complicating. Whenever there is more than one subject in a sentence and the subjects are connected by a conjunction such as 'and', the verb used is always a plural verb.

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.        

which sentences ? that question is incomplete