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Study Design 101

A similar study can be performed among preterm infants without access to maternal breast milk, randomizing these infants to donor human milk or formula Figure 4. To ethically conduct a randomized controlled trial, equipoise must exist between the investigative arm and the control arm.

Equipoise is a situation in which it is not known which of the 2 possibilities eg, donor milk or formula is more likely to achieve the outcome variable neurodevelopmental outcome at 2 years. The patients enrolled in a randomized controlled trial may not fully represent the target population for the intervention.

How to rate Risk of bias in Observational studies – Get Help, and give us Suggestions and Feedback

Before-after study design historical controls 12 : If randomization is not possible or will not be used, one possible study design is to compare people who received care before a program was established donor milk availability for VLBW infants in a NICU with those who received care after the program or health care measure became available on the outcome variable incidence of NEC. Provide suggestion although not conclusive demonstrating the effectiveness of a health care intervention. Data obtained in each of the 2 periods are frequently not comparable in terms of quality or completeness.

Often data collected after implementing the program is complete and of research quality. However, data collected before program implementation is from health care records designed and used only for clinical purposes. Other factors may have changed over time. The NICU may have instituted a bronchopulmonary dysplasia BPD prevention bundle or a central catheter—associated bloodstream infection bundle that may have an effect on primary outcome variable NEC.

Crossover study : Study designs typically are between group parallel comparisons Figure 4 or within group comparison eg, incidence of feeding intolerance before and after starting fortification. The crossover design has features of within- and between-group designs.

In addition to planned crossover design, patients may also cross over in parallel design trials unplanned crossover. Unplanned crossover and intention to treat 12 : Figure 5 shows the design of a randomized trial of Nissen fundoplication, comparing it with medical therapy for gastroesophageal reflux disease. Randomization is performed after informed consent is obtained. Some parents of infants assigned to Nissen fundoplication may have second thoughts after randomization and refuse surgery.

In addition, some patients assigned to medical therapy may deteriorate and require Nissen fundoplication. This is called unplanned crossover. Ideally, analysis is performed on the basis of intent to treat: patients assigned to the surgical arm at randomization are analyzed as the surgical group although some patients did not get surgery , and patients randomized to medical arm are analyzed as the medical group although some patients had surgery.

Figure 5.


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The primary outcome variable is daily weight gain. After 1 week, fortification is stopped in both groups for a week washout period so that carryover effects of one fortifier to the other group are avoided. After 1 week of the washout period, group B is fortified with liquid fortifier, and group A is fortified with MCT oil crossover period.

Weight gain is compared between groups A vs B and also within each group Figure 6. Figure 6. Planned crossover studies are attractive and useful because they evaluate within-group and between-group comparisons Figure 6. Crossover design removes between-patient variation and requires fewer patients. Order in which therapy is given may elicit psychological responses and difference in physiologic maturity with increasing postnatal age may influence response.

Open-label study is a type of clinical trial in which the researchers and participants or parents know which treatment is being administered. This contrasts with single-blind and double-blind designs. An open-label study may still be randomized. Strengths: A blinded trial is not possible in certain circumstances involving surgery abdominal drain vs laparotomy for NEC or intestinal perforation or physical intervention optimizing cooling trial for hypoxic ischemic encephalopathy [randomization to Limitations: A blinded trial is regarded as being less subject to bias than an open trial because it minimizes the effect of knowledge of treatment allocation on reporting of outcomes.

If the hypothesis was formulated after the data were analyzed, it is known as a post hoc hypothesis. Because spurious associations may be present just by chance, post hoc analysis should only be hypothesis generating and should be tested in future trials to confirm the effect seen. Strengths: A clinically relevant association may be detected during post hoc analysis. In a study evaluating inhaled nitric oxide in pulmonary hypertension, a finding associating respiratory alkalosis with later-onset sensorineural deafness may be detected by post hoc analysis.

As noted above, however, this hypothesis ideally should be tested in a future trial before an association can be confirmed. However, such a trial may not be ethically appropriate.

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Limitations: In addition to caution regarding interpretation of a finding on post hoc analysis, if the number of analyses increases, some positive results may be due to chance. The risk of erroneous conclusion increases with post hoc analysis. Subgroup analyses are defined as comparisons between randomized groups in a subset of the trial cohort. The main reason for performing these analyses is to discover effect modification interaction in subgroups, for example, whether inhaled nitric oxide is more effective in reducing the incidence of death or BPD in preterm infants with birth weights greater than 1, g compared with infants with birth weights of 1, g or less.

To preserve the value of randomization, subgroups should be defined by measurements that were made before randomization. The subgroup effect should be one of a small number of hypotheses tested. If a large number of hypotheses are tested, some of the statistically significant findings may be due to chance alone. To discover whether the effect of treatment is different based on sex, gestational age, or birth weight. Being smaller than the entire trial population, there may not be sufficient power to find important differences.

A systematic review is the assembly, critical appraisal, and synthesis of all relevant studies addressing a clearly formulated clinical question and incorporating strategies to minimize bias. A comprehensive search of the literature is performed to identify, select, and critically evaluate all relevant research using systematic and explicit methods and to collect and analyze data from all relevant studies included in the review. Statistical methods meta-analysis may or may not be used to combine the data in the included studies. Systematic reviews are retrospective analyses and, similar to other retrospective research, are subject to bias.

To limit bias, the clinical question should be clearly stated and explicit and systematic methods that can be reproduced by others should be used throughout the process. The inclusion and exclusion criteria for the search and selection of studies, the subgroup analyses planned including the anticipated direction of subgroup effect , and sensitivity analyses planned should all be specified a priori. The purpose of a systematic review: Systematic reviews summarize the available evidence relating to a specific clinical question.

In addition to providing an overall estimate of treatment effect to guide clinical decisions, systematic reviews can also help to inform research by identifying the areas of uncertainty requiring further study and guide policy decisions based on the entire body of evidence. Advantages of adding a meta-analysis to a systematic review and interpreting the results of a meta-analysis. A meta-analysis is a statistical method to pool effect estimates of individual studies and provide an overall estimate of treatment effect. The results of a meta-analysis are presented in a forest plot Figure 7.

The forest plot has a horizontal scale that displays the possible values of treatment effect.

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Use of self‐controlled designs in pharmacoepidemiology

If the effect estimate is a ratio eg, odds ratio or risk ratio , the scale is logarithmic. Alternatively, if the effect size is presented as a difference eg, risk difference or difference in means , the scale is linear.

What is a clinical case series

On the logarithmic scale, the value of 1 lies on the line of no effect, whereas on the linear scale, the value of 0 lies on the line of no effect. The vertical line in a forest plot is called the line of no effect, and if the confidence interval CI crosses this line, it indicates no difference in outcomes in the treatment and comparator arms.

Figure 7. On a forest plot, the treatment effect of each individual study is represented by a square. The point at the center of the square is the point estimate. The relative weight given to each study is represented by the size of the square usually determined by the study size , the CI is represented by a horizontal line that runs through the center of the block. The pooled effect estimate is represented by a diamond at the end of all individual study estimates. The point estimate of the pooled effect is at the center of the diamond, and the CI is represented by the width of the diamond.

If the diamond crosses the line of no effect, this indicates that after combining all relevant studies, there is no significant difference in effect in the treatment vs comparator.

The clinical case report: a review of its merits and limitations

It is not always appropriate to pool results of individual studies in a meta-analysis. Data from different studies can be combined when the studies address a common question and measure and report outcomes similarly. Alternatively, if the studies are very dissimilar or if the studies are at a high risk of bias, leading to a low confidence in the estimate of effect, the data should not be statistically pooled.

The advantages of adding a meta-analysis to a systematic review can be summarized in the following points:.