Master
Master
For literature studies at the master's level, the full methodology of systematic literature reviews is often used. However, we recommend that the research question to be as precise as possible so that the number of references allows for a feasible project. In practice, this means that together with a supervisor, you seek to limit the project so that it corresponds to a manageable amount of literature.
Be aware that the literature search is only a small part of the total methodology to be used in conducting a systematic literature review.
In our examples, we present a sensible solution for a literature search for the research question. We want to emphasize that there is never a definitive answer for literature searches, and that multiple solutions can work.
Example from Medline & Embase
We take the following project as our example:
What is the best treatment for accidental hypothermia?
Step 1: From research question to searchable terms
A crucial method to employ before identifying the relevant search keywords for your project is to ask yourself the following question: What main elements from my project do I want to find in relevant publications?
The main elements in our example are:
- Hypothermia
- Intervention
- Cause of hypothermia
To systematize these main elements and find the correct scientific terms, we recommend setting the main elements as headings for separate "boxes". This provides a clear overview of the individual search keywords and helps you conduct the search in a systematic and structured manner.
Step 2: Find search keywords for each main element
Now you can start the process of finding the correct scientific search keywords. There are many different methods to use. For example, you can ask your supervisor, a study colleague, consult a medical dictionary, ask your supervisor for relevant articles, and look these up in a relevant database, where you can easily find the controlled search keywords with which these are indexed.
In our example, we utilize the expertise of the supervisor and base our search on relevant articles recommended by the supervisor. In the initial work of finding relevant search keywords and getting an overview of the terminology used within the chosen theme, you will almost always find that multiple search keywords can be used for each main element. This step in the 5-step method of building and conducting a structured and systematic search always takes time and requires a lot of work. Therefore, set aside plenty of time!
In our example, we find the following words:
- Hypothermia, Cold temperature, Cold exposure, Body temperature
- Extracorporeal Membrane Oxygenation, Rewarming, Heart-lung machine, Cardiopulmonary Resuscitation
- Causes of hypothermia caused by accidents can be many. Here, it is important to try to cover as many eventualities as possible. Discuss with your supervisor or others who may have expertise in the area and read relevant publications. In our example, we find: Accidents, Avalanches, Mass Casualty Incidents, Emergencies, Aviation accidents, Warfare, Drowning.
We now fill these words into our "boxes," and get the following setup for our search:
For a literature study on master's level, you should run your search in the most relevant database(s). If you are unsure which database(s) are the most relevant, you should discuss this with your supervisor or contact the University Library. For this example, Ovid Medline, and Ovid Embase Classic+Embase, are the most relevant databases.
We start first in Ovid Medline. The first thing we do in Medline is to look up all the translated search keywords under each main element in the controlled search vocabulary (MeSH – Medical Subject Headings – Medical search keywords/subject terms). This is a very important step in the process because we should always use controlled search keywords if such exist for the relevant main element. If you are unsure what a controlled search vocabulary is and what advantages it offers, you should read our page about controlled search keywords again.
In Ovid's interface for the three databases Medline, Embase, and PsycINFO, you should always conduct your search under the 'Advanced Search' tab. Here exemplified with the first search keyword in our first box, 'hypothermia'. In the search window above, you will see that 'Keyword' and 'Map Term to Subject Heading' are checked. This is a standard setup in the Ovid interface and ensures that Medline searches in the controlled search vocabulary for the word written in the search window. When we then press 'Search', we get the window below, where we now see that the mapping algorithm suggests the controlled search keyword Hypothermia (MeSH-term).
You should always check the definition of the controlled search keyword that is suggested. You do this by clicking on the information symbol to the right of your controlled search keyword. You find this under the heading 'Scope'.
Here you read the objective definition of this controlled search keyword.
Note: Below the heading 'Used for:' in this window, you will always find an overview of synonyms that can be used as free search keywords. These should also be considered for use in your search. In connection with systematic literature reviews, the search should be improved by also using some synonymous free search keywords. Indexing of articles with controlled search keywords, such as MeSH terms, takes time. There may therefore be delays of months, or longer. In addition, errors can occur in the indexing. Therefore, it is always good to build a literature search that also includes one or more free search keywords, in addition to the controlled search keywords.
To ensure that you include any controlled search keywords further down in the hierarchical structure of the controlled search vocabulary, it is important that you check the box 'Explode' (see our page about controlled search keywords).
We then click 'Continue', and are forwarded to an overview of subheadings. Here you check off 'Include All Subheadings', unless you want to focus on one or more of these.
Then we click on 'Continue' and you now include all references that are indexed with the controlled search keyword 'Hypothermia'.
By clicking on the arrow symbol to the right of 'Search History', at the top left of the main menu, you can expand/collapse the search history. We see that 'Hypothermia', exploded, yields 15,273 references (exp Hypothermia/ 15,273). We have now found the controlled search keyword for 'Hypothermia'. We mark this in our box diagram.
We then continue by checking if the search keywords below each main element exist as controlled search keywords. We use the same procedure for each of the search keywords we have found so far.
Step 3: Build the first search
Now, you have found controlled search keywords for all the main elements of your project.
These can be found under 'Search history' on Medline's main page. Note that the controlled search keyword for 'Freezing' is not included. The reason for this is that it is a controlled search keyword that falls under 'Cold Temperature' in the hierarchical structure of the controlled search vocabulary, and it is therefore included in the search when we check 'Explode' 'Cold Temperature'.
You are now ready to combine the controlled search keywords for a first literature search. Controlled search keywords from each main element are combined with OR in the following way. Check each of the controlled search keywords you have found in the first box.
Then click on the button with the Boolean operator OR under the search history, and you will see that these controlled search keywords combine in line 15 in 'Search history'.
Repeat this for the controlled search keywords you have found in the next two boxes. One box at a time!
You will then find the combined search keywords from the three boxes, in lines 15, 16, and 17 respectively.
You are now ready to combine these three separate searches with AND. Check off lines 15, 16, and 17. Then click on the Boolean operator AND, in the search history. You now see that your first search with controlled search keywords gives you 176 references (line 18).
Step 4: Improve the search
The search we have conducted so far only used controlled search keywords for each main element from the project title. You can improve the search by using some synonymous free search keywords in what we call a text word search. Indexing articles with controlled search keywords, such as MeSH terms, takes time. Hence, there can be a delays of months, or longer. Furthermore, small errors can occur in the indexing. Therefore, it's always good to build a literature search that also includes one or more free synonymous text words, in addition to the controlled search keywords.
Let's take our example:
Let's take our example:
Where can you find synonymous free search keywords for these controlled search keywords?
One of the most important tools you have is actually to start by familiarizing yourself with the terminology of the scientific field you are working with. This is best done by starting to read articles and writing down the terminology used by individual research groups.
Another very useful tool is to look up the individual controlled search keyword you have found in the controlled vocabulary. In Medline, this is the MeSH database, which we have previously used.
From the previous example above, you remember that we could see the definition of each controlled search keyword, here exemplified with 'Hypothermia'. Under 'Used for' you find synonymous text words for the controlled search keyword.
We decide to go for precision here and note 'accidental hypothermia*' as a free search keyword.
Text word searches are done in selected search fields. The most common search fields we use in Medline are: 'Title', 'Abstract', and 'Keyword Heading' (authors' own keywords). We note this in our search box setup, where we mark the three fields we will search in, with ti (Title), ab (Abstract), and kw (Keyword Heading). The syntax for the synonymous text word then becomes accidental hypothermia*.ti,ab,kw. (Note the use of period and comma, and that no spaces are used.)
We use the same method for all the controlled search keywords. In addition, we should use other free search keywords we have picked up from various sources. We use the same syntax as described above.
By working in a structured and systematic way as we have now demonstrated, we get the following box setup for our search:
What remains now is to conduct this search in Medline. A good tip is to do this in a structured and clear way. This means starting with the search keywords in the box on the left, and searching each keyword individually. We start by bringing in 'Hypothermia' as a controlled search keyword. Then we click on the 'Search Fields' tab (to the left of 'Advanced Search'). Here we get an overview of all the metadata fields that each reference is indexed in. We also see that 'All Fields' is checked.
We uncheck 'All Fields' and enter our first synonymous text word: 'accidental hypothermia*', and check in 'Search Fields': 'Title', 'Abstract', and 'Keyword Heading'. Then we click on search.
We see that in Medline there are 982 references that have the text word 'accidental hypothermia*', in title and/or abstract and/or authors' keywords.
A practical tip is to do this 'Search Fields' search in the 'Advanced Search' screen. This saves you from clicking around on the various fields in the 'Search Fields' screen. You have now learned that the field code for the three search fields is ti (title) ab (abstract) kw (keyword heading). You enter these directly into the search window of 'Advanced Search', with this syntax: accidental hypothermia*.ti,ab,kw.
When you click on the 'Search' button, you see that the result is the same (line 3 in the search history) as you got by going into 'Search Fields' (line 2 in the search history).
We now use the box setup we have come up with, and enter and search for one search keyword at a time so that we get a structured and systematic search history.
We are now ready to combine our improved search, by doing exactly the same as in Step 3. We first select all the search keywords from the first box under the main element 'Hypothermia'.
Then we combine with OR. We repeat this for the search keywords in the next two boxes. We now see that we have 3 lines, 29, 30, and 31, where the search keywords for each main element are combined with OR. Now we can conduct our expanded literature search by combining these with AND.
We now have 723 references, compared to our first search which yielded 176 references. We have thus conducted a more sensitive search in our chosen database. As a master's student, you must, together with your supervisor, make some qualified considerations about the scope of your first search. If there are too many irrelevant references among these 723, you can remove the abstract field, a field that can add some "noise" to the search result in the form of irrelevant literature, and only search in the title and authors' keywords. In that search, you use the same setup, but exclude the abstract (ab). This means that the first text word search that we had in this example, accidental hypothermia*.ti,ab,kw, will now look like this: accidental hypothermia*.ti,kw. We then end up with 441 references. Thus, a more specific search.
Step 5: Adapt the search to other databases
For most master's projects within health sciences, the databases Medline, Embase, PsycINFO (on Ovid's interface), CINAHL, Web of Science, and Cochrane will be relevant databases. If you are unsure which database(s) to choose, you should speak with your supervisor or contact the University Library. It is rarely sufficient to search in just one database for a literature review at master's level.
The interfaces of these databases can feel very different, but the search methodology explained above can be used in any of these reference databases.
We will now convert our example search from Medline to Embase Classic+Embase. From our example, you now know that Medline uses MeSH terms to index articles so that we can easily find relevant references. Embase, which is also on Ovid's interface, uses a different controlled search vocabulary, called Emtree. When moving from one database to another, it is therefore necessary to look up the controlled search keywords you found in the database you first searched in. In our example, we must find Emtree terms that correspond to the MeSH terms we used in the Medline search.
After you have conducted and saved your search in Medline, click on the 'Change' link to the right of '1 resource selected'.
A new window will open where you select Embase Classic+Embase. After that, you have the option to click on 'Continue' or 'Run Search'. We recommend that you click on 'Continue'. The reason for this is that 'Run Search' will conduct your Medline search in Embase, without you having quality assured whether the controlled search keywords from Medline are used as controlled search keywords in Embase. By clicking on 'Continue', you start a new search in Embase, and you will then have full control over each step in the search process as described above.
To illustrate that there can be differences between the naming (and level in the hierarchical structure of the controlled search keywords), we take the controlled search keyword 'Extracorporeal Membrane Oxygenation' that we found as a MeSH term in Medline. We enter this into the search window in Embase.
When we click on the 'Search' button, we see that the controlled search keyword 'Extracorporeal Membrane Oxygenation', from Medline, only matches 'extracorporeal oxygenation' as a controlled search keyword in Embase.
To ensure that the definition of the proposed controlled search keyword 'extracorporeal oxygenation' matches the definition we have from the MeSH term, we click further on the information icon under 'Scope'. Here we can read that the proposed Emtree term 'extracorporeal oxygenation' is defined exactly like the MeSH term 'Extracorporeal Membrane Oxygenation'. Additionally, this Emtree term has a very long list of synonymous text words, which you find under 'Used For'. These should be considered for inclusion in the search, as synonymous free search keywords.
If you find new free search keywords that you have not used in previous searches when you, as in this example, move from Medline to Embase, you should consider including these in your original search setup, and conduct a new search in Medline. We do not demonstrate this in our example, but this is something you need to be aware of. The point is that when transferring a search from one database to another, the searches should be as similar as allowed by the differences in search vocabulary and syntax.
Just like in Medline, you can search in title, abstract, and authors' keywords in Embase, using these field codes: ti,ab,kw.
Below you find the same search as in the example from Medline, but now conducted in Embase. Here we have ONLY used the search keywords that we used in the example from Medline (we have NOT added more free search keywords). Note that most controlled search keywords have changed names, compared to the controlled search keywords from the search in Medline (you find that these start with exp). This is explained by the fact that Embase uses the controlled vocabulary Emtree. Also note that we do not use the controlled search keyword 'Freezing' from our first search box here either. In Embase, 'Freezing' is defined as the process of freezing down food items, etc.
When we combine the three main elements in our box search setup, we see that Embase delivers 1519 references, compared to the 723 references we got in Medline.
