List three reasons why a study may be considered invalid

Three things why a study can be considered invalid might be:

Lack of Research: if a study does not achieve the requirements of the research method then it is not valid.

Biased opinions: if a study is based on one specific way of the researcher then in characterized as invalid.

No evidence: if the study does not have enough or not at all evidence of what it was investigated then it cannot be reliable and so, invalid.

1. - Improper Information

2. - Not doing trials again to prove it.

3. - Because most studies are pushed to be published to it can have missing information.