Cambridge Grammar of English Hardback with cd-rom a comprehensive Guide

Should and ought to

Should

allows the writer/speaker to describe desired or ideal situations. It is less

strong than must:

However, to assess different advantages and disadvantages in other

circumstances, the chosen method 

should be examined critically before use.

(compare the stronger: … the chosen method must be examined …)

Ought to

is occasionally used in this way in academic style, but is much less

frequent than should:

Our use of the term ‘stable’ 

ought to be defined here.

Should

is used to hedge conclusions and predictions, but it expresses confidence

in the probability that a situation will occur in a particular way:

The overall agreement of the results 

should allow us to accept them with some

confidence. 

(compare the more direct: … the results allow us to accept …)