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Table 3 Risk of bias of quantitative studies based on the EPOC tool

From: Specialist antenatal clinics for women at high risk of preterm birth: a systematic review of qualitative and quantitative research

Study/year Selection bias Allocation to intervention Performance bias Baseline differences in characteristics Baseline differences in outcomes Contamination Attrition bias Selective reporting Other bias
Randomized controlled trials (RCTs)
1. Hobel et al. [25] Low: Cluster randomization with a restricted block Low: cluster randomization. Low: women were not aware of their intervention status nor the clinics teams Low: participants were comparable with respect to age, marital status, gravidity, parity and preterm birth rate Low: no difference between the groups with regards to high risk preterm problems at baseline Low: intervention was provided on a clinic-basis rather than patients Unclear: the number of women who left the study was not reported Low low
2. Main et al. [22] High: a random numbers table was used for the first 479 participants then the second sample of464 women was divided into groups by birthday date High: women’s’ date of birth was used to allocate women to intervention or control Low: In the control group neither the doctors nor the women were made aware they were at high risk of preterm birth. Low: no differences between groups with respect to maternal age, gravidity, parity, previous preterm deliveries, and gestational age at first visit High: More women with previous preterm birth were assigned to the intervention. High: 8 women from the control group transferred to the clinic High: insufficient reporting on the rate of attrition. Unclear Low
3. Iams and Johnson [21] Unclear Unclear Unclear Unclear Unclear Unclear Unclear Unclear Unclear
4. Goldenberg et al. [24] Low: randomization by a randomization officer Unclear: allocation to intervention or control method was not reported Low: nobody was aware of the intervention status Low: No significant difference between high-risk group and high-risk control with regard to number of birth, race, age and parity Low: no differences of previous preterm birth and multiple pregnancies between the two groups low Unclear: the number of missing women was not reported Low Low
5. Mueller-Heubach [23] Low: participants were selected to intervention and controlled randomly Unclear: method of allocation to intervention or control was not reported High: nurses were aware of the intervention status of the participants Low Low High: a historical control was used in the analysis Unclear low low
Cohort studies
1. Herron et al. [26] High High High Low Unclear Low Low Low Low
2. Manuck et al. [27] High High High Low Low Low Low Low Low
3. Karkhanis et al. [28] (abstract) High High High Unclear Unclear Low Unclear Unclear Unclear
4. Burul [29] (abstract) High High High unclear Unclear Low Unclear Unclear Unclear
5. Cohen et al. [30] (abstract) High High High Unclear Unclear Low Unclear Unclear Unclear