Johns Hopkins Nursing Evidence-Based Practice Thrid Edition: Model and Guidelines

(^134) Johns Hopkins Nursing Evidence-Based Practice: Model and Guidelines, Third Edition
Quasi-Experimental Studies (Level II Evidence)
The evidence gained from well-designed, quasi-experimental studies is lower than
that of experimental studies. Quasi-experimental studies are indicated when ethi-
cal considerations, practical issues, and feasibility prohibit the conduct of RCTs.
For that reason, the evidence rating for a well-designed quasi-experimental study
is Level II (refer to Table 6.7).
As with experimental studies, threats to internal validity for quasi-experimental
studies include maturation, testing, and instrumentation, with the additional
threats of history and selection (Polit & Beck, 2017). The occurrence of external
events during the study (threat of history) can affect a subject’s response to the
investigational intervention or treatment. Additionally, when groups are not as-
signed randomly, preexisting differences between the groups can affect the out-
come. Questions the EBP team may pose to uncover potential threats to internal
validity include, “Did some event occur during the course of the study that may
have influenced the results of the study? Are there processes occurring within
subjects over the course of the study because of the passage of time (maturation)
rather than as a result of the experimental intervention? Could the pretest have
influenced the subject’s performance on the posttest? Were the measurement in-
struments and procedures the same for both points of data collection?”
In terms of external validity, threats associated with sampling design, such as pa-
tient selection and characteristics of nonrandomized patients, affect the general
findings. External validity improves if the researcher uses random selection of
subjects, even if random assignment to groups is not possible.
Nonexperimental and Qualitative Studies (Level III Evidence)
The evidence gained from well-designed, nonexperimental and qualitative studies
is the lowest in the research hierarchy (Level III). Questions of internal validity
do not apply when reviewing descriptive designs (quantitative or qualitative).
When looking for potential threats to external validity in quantitative non-
experimental studies, the EBP team can pose the questions described under ex-
perimental and quasi-experimental studies. In addition, the team may ask further