Reviewer ’s report  
 
Title: Predictors of cervical cancer screening service utilization among commercial sex workers 
in North West Ethiopia: A case -control study.  
 
Version: 1 Date: 20 Jul 2019  
 
Reviewer: Paolo Giorgi Rossi  
 
Reviewer's report:  
 
The paper has been improved. Some of the answers do not address the issues, I am sorry for 
being not clear.  
 
The main issue that has not been addressed is the problem of adjusting for all variables even 
when they  are clearly on the same causal chain. Assessing that there is no collinearity does not 
exclude that part of  the effect of STI is explained by increasing the number of visit. Collinearity 
would be the extreme case in which all the variability due to STI i s explained by number of visit 
and vice -versa. The possible existence of a mediating effect first of all implies that the 
researchers make the hypothesis of a direction of the causality, and then only part of  the effect 
of STI could be explained by increa sing the visit and another by increase knowledge due to 
counselling for STI, and also the opposite is not needed (i.e. that the variability of visits is due to 
STI).  On the other hand if there is no association at all between the variables, they cannot be  
confounders each other so it is not useful to build a multivariate model. Adjusting STI effect for 
number of visit simply would hide part of the effect of STI that is mediated by visit. The best 
option would be a formal mediation analysis, but a simpler s olution could be to add a column to 
table 5: COR, OR adj for age and all the variables that cannot stay on the same pathway 
(sincerely I do not see any other than duration of sexual working and history of abortion), and 
finally the fully adjusted model OR.  If OR does not chage you can argue that the effect is not 
mediated by all  the others variable (mainly number of visits) if it changes you cannot say that 
there is no effect adjusting, but that probably the effect is mediated by the number of visits and 
the provider recommendations.  
 
The second issue that has not been answered correctly is the issue of prevalence of pap uptake. 
This case control study start form the assumption that all "cases", i.e. sexual workers with Pap 
have been included in the study. T herefore, if you have even approximate denominator of sexual 
workers attending the clinic you should have an approximate measure of the prevalence (this is 
the case as reported in the methods section "Study setting and participants"). Please note that in 
previous version of the abstract there was  no mention of  the sample size, so the answer to my 
comments should be, first of all to acknowledge this point. Then I also disagree on the concept 
that case control cannot give prevalence at all: if the condition  of including all cases (that is an 
assumption for unbiased case control study) and you have a defined study base (you included 
only already registered cases in your clinic, so eligible population is registered women attending 
the clinic) the prevalence is  known.   
 
Are the methods appropriate and well described?  
If not, please specify what is required in your comments to the authors.  
 
Yes 
 
Does the work include the necessary controls?  
If not, please specify which controls are required in your comments to the authors.  
 
Yes 
 
Are the conclusions drawn adequately supported by the data shown?  
If not, please explain in your comments to the authors.  
 
Yes 
 
Are you able to assess any statistics in the manuscript or would you recommend a n 
additional statistical review?  
If an additional statistical review is recommended, please speci fy what aspects require further 
assessment in your comments to the editors.  
 
I am able to assess the statistics  
 
Quality of written English  
Please indicate the q uality of language in the manuscript:  
 
Acceptable  
 
Declaration of competing interests  
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