In a seminal, instant classic article John P. A. Ioannidis shows
Why Most Published Research Findings Are False.
He essentially argues that the only valid findings are of meta-analysis where multiple reproducible studies are used as the basis of a larger study of these studies.
Even Richard Horton, editor of The Lancet, says:
up to half of all research may be false:
...
The case against science is straightforward: much of the
scientific literature, perhaps half, may simply be untrue.
Afflicted by studies with small sample sizes, tiny effects,
invalid exploratory analyses, and flagrant conflicts
of interest, together with an obsession for pursuing
fashionable trends of dubious importance, science has
taken a turn towards darkness. As one participant put
it, “poor methods get results”. The Academy of Medical
Sciences, Medical Research Council, and Biotechnology
and Biological Sciences Research Council have now put
their reputational weight behind an investigation into
these questionable research practices. The apparent
endemicity of bad research behaviour is alarming. In their
quest for telling a compelling story, scientists too often
sculpt data to fi t their preferred theory of the world. Or they
retrofit hypotheses to fi t their data. Journal editors deserve
their fair share of criticism too. We aid and abet the worst
behaviours
...
There is a crisis in science, particularly the lack of reproducibility.