a diagnosis of constipation is made when a person's number of bowel movements per week first drops to under

The correct answer is 3.

According to the ROME III (2006), the criteria for functional constipation is that (1) loose stools are rarely present without the use of laxatives, (2) insufficient criteria to meet irritable bowel syndrome, and (3) must include 2 or more of the following:

1. Straining during at least 25% of defecations
2. Lumpy or hard stools in at least 25% of defecations
3. Sensation of incomplete evacuations in at least 25% of defecations
4. Sensation of anorectal obstruction/blockage in at least 25% of defecations
5. Manual maneuvers to facilitate at least 25% of defecations
6. Fewer than 3 defecations per week

Constipation is a delay or difficulty of the evacuation of less frequent stools, less abundant, harder than normal: less than 3 stools per week.
Frequency is not the only element of diagnosis. In constipation, the stool is hard, difficult to expel. And can be painful. And in diarrhea, the stools are soft, even liquid, and the intestines can be painful.