Review for 10.1186/s12876-020-01586-4 (2021)

Reviewer’s report
Title: 
Clinical features and outcomes of combined hepatocellular carcinoma and
cholangiocarcinoma versus hepatocellular carcinoma versus cholangiocarcinoma
after surgical resection: A propensity score matching analysis
Version: 
1 
Date: 
06 Aug 2020
Reviewer's report:
This paper includes comparison of three kinds of similar primary hepatomas;
hepatocellular carcinoma(HCC), combined HCC-cholangiocarcinoma and
cholangiocarcinoma in terms of prognosis. There were many published papers
regarding comparison among three different cancers. 
However propensity score
matched(PSM) analysis of three groups are unique study even though the result
is suspected as expectation on the basis of clinical experience and published
papers. One of the most interesting findings in PSM was that OS and recurrence
rate in the cHCC-CC group is not significantly different from those of HCC or
CC group, whereas it is significantly different without PSM. 
Criticisms: The most important finding in this study is comparison between
cHCC-CC and HCC, and between cHCC-CC and HCC after PSM. Then, after
PSM, the number of both group should be similar or 1 : 1, 1 : 2. The number is
too much different, n=16 vs n=51. There is high possibility that the number
discrepancy make bias, so that result in no difference statistics between group.
Authors' explanation about this is mandatory in the discussion. 
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Is the study design appropriate to answer the research question (including the use
of appropriate controls), and are the conclusions supported by the evidence
presented?
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