  As experimental methods become increasingly popular in crowd dynamics, the
question of experimental validity becomes more critical than ever. For the
purposes of evaluation of crowd experiments, it is paramount that we
distinguish between different dimensions of their validity. Here, we
differentiate between four types of experimentation in crowd dynamics based on
their underlying purpose of investigation. This includes experiments whose main
aim is (i) testing a behavioural theory/hypothesis, (ii) exploring empirical
behavioural regularities; (iii) assessing collective (system-level) performance
rather than individual behaviour (iv) calibrating and/or validating
pre-existing models. While delineating various dimensions of external/internal
validity as they relate to crowd experiments, we argue how the importance of
different validity dimensions varies depending on investigation purpose. This
could provide clarity about circumstances where artificiality of laboratory
settings is justified as opposed to experiments for which higher levels of
contextual fidelity should be given more weight. It is critical that
investigators (a) be conscious of the trade-off between controllability and
realism in their experiment design and (b) identify potential factors that can
jeopardise validity of their experiments and mitigate them. Inferences made
from field data do not invariably supersede those of experiments on the issue
of validity. Depending on the purpose of investigation, maintaining internal
validity sometimes necessitates laboratory settings and justifies
artificiality. In the absence of adequate control over the causal relationship,
the question of external validity will be futile. Therefore, striking a
reasonable balance between internal and external validity of crowd experiments
should be made a major design consideration.
