The gold standard is blind, independent replication. However, this almost never happens because it costs a similar amount (of materials and time) and there is less benefit to the researcher who repeats the experiments, so there is little incentive. Next down would be independent replication by another group, then replication within the same group. The bare minimum is that the experiment be described in sufficient detail to be reproduced. For example, the 'Instructions to Authors' in the Journal of Infection and Immunity (which is fairly typical) states:
the Materials and Methods section should include sufficient technical
information to allow the experiments to be repeated.
Reproducibility is one of the most important issues when describing or reviewing a scientific paper (ref). Knowing that a result is less likely to be attributable to a chance combination of uncontrolled parameters, or a mistake on behalf of the scientist conducting the experiment or recording the experimental conditions, increases confidence that the result seen is a 'genuine' phenomenon. As Karl Popper, the founder of the scientific method, put it:
Non-reproducible single occurrences are of no significance to science.
(Popper 1959, The logic of scientific discovery. Hutchinson, London, United Kingdom.)
Unfortunately, most journals place a high premium on novelty and therefore it is more difficult to publish reproductions of previous studies, and when these are published they are often in less important journals. Funders, similarly, direct reviewers to score grant applications based, amongst other things, on novelty. This greatly reduces the incentive to replicate published studies - if you can't get funding to do something and you wouldn't be able to publish it anyway, it won't get done. Two other major and common obstacles to independent validation are:
- that it requires the involvement of another lab with the same resources and skills, which might be very rare;
- For experiments involving animals specifically, there is an ethical requirement to minimise the use of experimental animals.
The lack of reproducibility of experiments is a problem in most areas of science and awareness of this is growing. In 2012 this prompted the Reproducibility Initiative, where researchers can pay a fee for blind, independent replication prior to publication.
This sounded like a step in the right direction at the time but as far as I know awareness of the initiative remains fairly low, uptake has not been great and 'proper' reproduction remains very rare indeed.