Soumya Ranjan Das To your first question, no -- it is not necessary to include negatively-phrased items in an instrument. This is generally done to help ensure participants are actually reading the items and answering truthfully. For that reason, it can be advantageous. However, negatively-phrased items do not (in and of themselves) make an instrument more valid.

As to your second question -- negatively-phrased items should NOT be an exact opposite of a positive item in the instrument. For example, you would not want to include both of these items:

Q1. I am happy

Q2 I am not happy

The reason you wouldn't want to include both is because Q2 is a redundant item. It's not providing any new information beyond Q1. Thus, if you wish to include negatively-phrased items, the goal should be to measure some new aspect, facet, or dimension of the underlying construct.

Can you include a negative statement? - Yes, provided you indicate in the scoring that the negatively worded item needs to be reverse-scored while totalling.

Is it necessary to include a negative statement? - No.

Using negatively worded (reversed) items in scales is an old-fashioned idea that was intended to somehow force people to pay more attention to their responses. But actual testing of this idea showed that it did not work and thus introduced weaker correlations. So, the present opinion is against using negatively worded items.

David L Morgan That is interesting. Would you happen to have a copy of the aforementioned papers you could share, or otherwise share a few references? Thanks!

To reverse or to not reverse Likert-type items: That is the question

Andreu Vigil-Colet, et al.

Using reversed items in Likert scales: A questionable practice. Javier Suárez-Álvarez, et al.