Selecting eligible studies
Nicholas Ngwili, ILRI
Online Training on Systematic Literature Review, ILRI Nairobi, 5-6 May 2020

Learning outcomes
o Describe the main stages involved in selecting studies
o Apply inclusion criteria to assess eligibility of study abstracts
and full text articles

Influential decision
“Decisions about which studies to include and which data to
extract are among the most influential that are made in the
review process – but they involve judgement”
Higgins J et al. Chapter 7. Cochrane Handbook for Systematic
Reviews of Interventions. Wiley & Sons.

Valid & reliable data collection
What you need:-
 Importance of clear, well-defined PICOS
 Pre-specified, explicit, reproducible protocol
 Measures to minimise bias and error

Steps
SCREEN SEARCH OUTPUT
 Merge search output and remove duplicates
 Identify studies that are potentially eligible for inclusion
ASSESS FOR ELIGIBILITY
 Retrieve full texts of potentially eligible studies
 Make decision to include studies by examining full texts
EXTRACT DATA
 Extract data of interest to review question

Step 1 Merge search output and remove
duplicates
What you may have:-
• Large sample of records (100s, 1000s, 10,000s..!)
• Duplicate records
• You will hopefully include all eligible studies as well as many
studies that do not meet inclusion criteria
• ‘Small’ % eligible for inclusion

Typical flow chart- Example From T.solium
SLR

Removing duplicates
First merge all searches into a manageable format e.g.
EndNote database or Mendeley or other reference management software
 Then can de-duplicate in two ways:
1. Using automated methods
2. Manually
 Note even after this process there may still be ‘duplicate’
publications of very similar data by the same research group
so need to be careful to exclude before meta-analysis

Step 2 Identify studies that are potentially
eligible for inclusion -screening
inclusion (screening)
o Each record in the search output is examined
o ‘Mark’ records of studies which are judged to be potentially
eligible
o Be over-inclusive
o subset of original search output left

To minimize bias and error...
 Two authors independently screen records
 Software allows datasets to be merged and discrepancies identified -
DistillerSR
 Discrepancies may result from either oversight or differences in
interpretation
 Discrepancies resolved through discussion/ referral to 3rd author
 Left with agreed subset of search output containing records of potentially
eligible studies

Step 3 Retrieve full texts of potentially
eligible studies
Aim to retrieve full texts of all potentially eligible studies
identified by screening
o Online
o Libraries
o Original authors
 Labor intensive, time consuming
 Cost

Step 4 Make decision to include studies by
examining full texts
 Decide to include studies by assessing full texts against
PICOS
 Only studies that meet all criteria will be included
 Consider translation of foreign language articles (cost, how
much of article needs to be translated?)

Good record keeping is essential!
 Document decisions
 Decisions may be queried later
 List of excluded studies with reason. It is a good idea to have a
hierarchy of reasons for exclusion, otherwise the different
reviewers may select different reasons
 Complete PRISMA flow chart as you go

References
• http://handbook.cochrane.org/

This presentation is licensed for use under the Creative Commons Attribution 4.0 International Licence.
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