External validity is the validity of generalized (causal) inferences in scientific studies, usually based on experiments as experimental validity. In Psychology, validity has two distinct fields of application 
Inferences about cause-effect relationships based on a specific scientific study are said to possess external validity if they may be generalized from the unique and idiosyncratic settings, procedures and participants to other populations and conditions Causal inferences said to possess high degress of external validity can reasonably be expected to apply (a) to the target population of the study (i. e. from which the sample was drawn) (also referred to as population validity), and (b) to the universe of other populations (e. g. across time and space).
The most common loss of external validity comes from the fact that experiments using human participants often employ small samples obtained from a single geographic location or with idiosyncratic features (e. g. volunteers). Because of this, one can not be sure that the conclusions drawn about cause-effect-relationships do actually apply to people in other geographic locations or without these features.
"A threat to external validity is an explanation of how you might be wrong in making a generalization. " Generally, generalizability is limited when the cause (i. e. the independent variable) depends on other factors; therefore, all threats to external validity interact with the independent variable.
In many studies and research designs, there may be a "trade-off" between internal validity and external validity: When measures are taken or procedures implemented aiming at increasing the chance for higher degrees of internal validity, these measures may also limit the generalizability of the findings. Internal validity is the validity of (causal inferences in scientific studies usually based on experiments as experimental validity. This situation has led many researchers call for "ecologically valid" experiments. By that they mean that experimental procedures should resemble "real-world" conditions. They criticize the lack of ecological validity in many laboratory-based studies with a focus on artificially controlled and constricted environments. Ecological validity is a form of validity in a research study External validity and ecological validity are closely related in the sense that causal inferences based on ecologically valid research designs often allow for higher degrees of generalizability than those obtained in an artificially produced lab environment. Ecological validity is a form of validity in a research study However, this is not always the case: Some findings produced in ecologically valid research settings may hardly be generalizable, and some findings produced in highly controlled settings may claim near-universal external validity. Thus, External and Ecological Validity are independent - a study may possess external validity but not ecological validity, and vice-versa.
Within the qualitative research paradigm, external validity is replaced by the concept of transferability. Qualitative research is a field of inquiry that crosscuts disciplines and subject matters. This page is about transferability in chemistry Transferability in economics also exists Transferability is the ability of research results to transfer to situations with similar parameters, populations and characteristics.