Table 2. Main types of quantitative research designs and their associated advantages and disadvantages.

Type of study design	Advantages	Disadvantages
RCT (parallel design)	• In well-constructed study, unbiased distribution of confounds • Blindness of assessment • Can provide temporal sequence of events • Statistical analyses facilitated by randomization	• The ethical issue of who receives a specific treatment and if that treatment can possibly do more harm than good • Designs costly and time-consuming • Compliance issues or participants lost to follow-up may occur, which affects validity • Randomization techniques may be faulty; however, established methods could account for proper randomization of patients • Through time, contamination between groups may occur and should be accounted for • Biases (preallocation, selection, performance, detection, exclusion, publication)
RCT (crossover design)	• All subjects receive treatment and serve as own controls • Error variance reduced and sample size needed is reduced • Blinding may exist	• All subjects receive placebo or alternative treatment at some point • Unknown or lengthy washout period • Not applicable in treatments that are associated with permanent effects
Cohort study	• Determines the incidence of developing the disease in both types of groups • Confounders can be evaluated and their influence upon the outcome can be determine • Can evaluate various outcomes, detect associations, analyze time relationships, monitor changes over time, and assess rare and unique exposures • Can establish the relations between antecedent events and outcomes • Compared with RCT, less expensive and easier administration	• Can be time-consuming (if prospective in nature, but retrospective cohort designs may reduce time and subsequent costs) • Difficult to follow the original sample group through time • Blinding is difficult and randomization not present • Poor sample sizes and short follow-up for rare diseases • Various hidden confounding variables may affect outcome
Case-control study	• Beneficial in studying rare diseases or diseases with long duration to develop outcome • Retrospective and thus inexpensive and quick studies to conduct if time is an issue	• Obtaining an adequate representative control group may be difficult • Sampling bias may exist where defining a homogenous disease group as well as control group could be problematic and contain confounds • Demographics of the groups in question may prevent generalizing results and increasing external validity • How subjects are recruited may be questionable • Obtaining data to determine exposure may be difficult and prove challenging/time-consuming • Due to the retrospective nature of the study, establishing a timeline when events occurred that may have contributed to the outcome is difficult, thus, obtaining consistent values of timing of events for both groups may not be probable • The study controls are selected from the investigator and can entail sampling bias, thus are not representative of the population as a whole and risk ratios cannot be analyzed • Recall bias by the participants may be present and dilute the validity of the results • Hidden confounders
Cross-sectional survey	• Inexpensive • Simple • Ethically sound	• Does not establish causality, but possible association • Potential for recall bias • Confounders unequally distributed • Unequal group sizes
Case series and case reports	• May provide insightful information into an area for further investigation • Provides insight for very rare diseases and their management	• Multiple and nonexistence of comparison group

Abbreviation: RCT, randomized controlled trial.

Source: Adapted from Samartzis D, Dominique DA, Perez-Cruet MJ, et al. Clinical outcome analyses. In: Perez-Cruet MJ, Khoo LT, Fessler RG, eds. An Anatomical Approach to Minimally Invasive Spine Surgery. St. Louis, MO: Quality Medical Publishing, Inc.; 2006:103–130.