Researchers in Spain conducted a study of the relationship between consumption of non-nutritive sweetened beverages and depression. They enrolled 3,000 young people aged 14 to 24 years drawn from the population registry offices of the city of Madrid and each of the 29 surrounding counties. Current intake of non-nutritive sweetened beverages and depression symptoms were ascertained at baseline and again at follow-up 4 years later. Depression was determined by interview with trained psychologists using the CES-D instrument. At follow-up, 792 individuals had reported at least one depressive symptom over the 4 year period and their frequency of non-nutritive sweetened beverage intake is provided in the table below. Assume that there was no loss to follow-up.

	Any symptom of depression at follow-up
Frequency of non-nutritive sweetened beverage intake	Yes (n=792)	No (n=2,208)
None	598	2,020
<1 time/month	38	48
3-4 times/month	39	37
1-2 times/week	36	27
3-4 times/week	35	28
5 times/week	46	48

What are the possible threats to external validity in this study? (Choose one best answer)

Low reliability of the instrument to assess depression

Residual confounding

The individuals in the study are not comparable to other populations with similar non-nutritive sweetened beverage intake

Interviewer bias

None of the above