At its simplest, scientific research is defined as a systematic inquiry into a phenomenon for the purpose of establishing knowledge.
It differs from public health advice, law and regulation which, in this context, aim to prevent the spread of a disease or promote healthy practices within a population.
The technical concept of validity within a research project is important because public health actions based on the evidence produced by scientific research will affect the whole population.
Study validity can be defined as the extent to which the findings of scientific research will be true in similar populations outside the study. The design and implementation of a research study ideally should maximise validity. Therefore conducting scientific research can take time, sometimes years.
And as an added complication, repetition of experiments (reliability) is required to be confident of knowledge claims (evidence) that might underpin public health action .
 Rychetnik, L, Hawe, P, Waters, E, Barratt, A & Frommer, M 2004, 'A glossary for evidence based public health.', J Epidemiol Community Health, vol. 58, pp. 538-545.