Reviewer’s report
Title: 
Time to first birth and its Predictors among reproductive-age women in
Ethiopia: inverse Weibull gamma shared frailty model
Version: 
0 
Date: 
26 May 2020
Reviewer's report:
The authors estimated time to first birth among Ethiopian women of
reproductive age and its associated factors. Although the research question is
scientifically sound, and the methods and statistical analysis are suitable, several
issues in the current version deserve attention before the manuscript may be
considered for publication: 
Background
- The introduction is focused on adolescent pregnancy and maternal death, thus
providing only partially rationale to the research question. It is well-known that
pregnancy in the extremes of reproductive age are related with increased
morbidity and mortality.
- The authors state that the evidence from local studies is scarce. However, most
determinants described have been identified consistently in the literature.
Actually, their results are similar to [1], which is cited in this manuscript. On the
other hand, other studies that found very similar results in Ethiopia 
are not cited;
for instance, [2] and [3]. So, it is difficult to understand the contribution of this
study to the field.
- Line 111. Usually estimates from survey studies take into account the
correlated nature of the data. It is unclear to this reviewer why incorporating that
in the analysis would be a novel contribution.
Methods
- Authors established beginning time at puberty (10 years), so time to birth
should be the difference of age at first birth minus 10. However, in the rest of the
text only age at first birth is used. 
- Given the scope of the journal, instead of the detailed technical description of
the model selection that extends from here to results, I would suggest to briefly
explain why a parametric model with shared frailty was used (why not a Cox
model and what is the role of frailty).
Results
- Eight figures and nine tables, plus two supplementary tables and one
supplementary figure are presented (!). I would suggest authors to focus on the
analyses that provide information to answer the research question. Other material
can be included as supplementary files. Especially, the extensive explanation on
model selection, goodness of fit, etc. suits better to an annex. 
Discussion
- The median age at the first birth in this study was 20 years, which is within the
optimal age for first birth (20-29). Furthermore, this median is higher than the
referred in background for Ethiopia (~18.5). How do authors interpret this
finding? 
- The associated factors are interpreted as determinants for pregnancy at an early
age. However, the relationship found would be the same for any age
(proportional hazards assumption). For instance, educated women would have a
lower risk of first pregnancy than those non-educated at age either 20 or 40.
- Although the authors obtained adjusted HRs, some associations are explained
by means of the same sociodemographic or contextual characteristics already
adjusted for (v.g, lines 574-593). Actually, in adjusted models the effects are
independent. 
Formatting/English language
This manuscript would benefit from an editorial review, due to several errors in
grammar and spelling. In addition, some lines are wordy, repetitive, not
understandable or disorganized. 
References
[1] Eshetu Gurmu and Dula Etana. Age at First Marriage and First Birth Interval
in Ethiopia: Analysis of the Roles of Social and Demographic Factors. African
Population Studies Vol. 28, No. 3, 2014
[2] Gizachew Assefa Tessema, Berihun Megabiaw Zeleke and Tadesse Awoke
Ayele. Birth Interval and its Predictors among Married Women in Dabat District,
Northwest Ethiopia: A Retrospective Follow Up Study. African Journal of
Reproductive Health. Vol. 17, No. 2 (June 2013), pp. 39-45.
[3] Ayele Gebeyehu Chernet, Dinberu Seyoum Shebeshi & Akalu Banbeta.
Determinant of time-to-first birth interval after marriage among Ethiopian
women. BMC Women's Health volume 19, Article number: 157 (2019).
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.
No
Are you able to assess any statistics in the manuscript or would you
recommend an additional statistical review?
If an additional statistical review is recommended, please specify 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 quality of language in the manuscript:
Not suitable for publication unless extensively edited
Declaration of competing interests
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