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The bachelor’s thesis often consists of a delimited literature study, derived from literature searches in one (or in some cases several) database(s). The choice of database(s) is clarified with the supervisor. The literature study typically includes methodological elements taken from the major systematic literature reviews but is not as comprehensive.

Example from PubMed

We base this on the following example of a project:

What is the first choice of pain-relieving medication for nerve root affection due to prolapse in the back?

Step 1: From research question to searchable terms

An important method to use, before finding the important search keywords in your own project, is to ask yourself the following question: What main elements from my own project do I want to find in relevant publications?

The main elements in our example are:

  • Pain relief
  • Nerve root affection
  • Prolapse

To find the correct scientific terms, we recommend setting the main elements as headings on their own "box". This gives you a good overview of the individual search keywords and helps you to 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 a supervisor, a study colleague, look up in a medical dictionary. You can ask the supervisor for relevant articles, and look these up in the releveant database(s), where you can easily find which controlled search keywords these are indexed with.

In our case we find the following medical terms:

  • Analgesics
  • Radiculopathy
  • Intervertebral disc displacement (it is important to use the correct term here, as the word ‘prolapse’ should not be used here. Prolapse can occur in many parts of the body while our project is only looking for prolapse in the back!)

We now fill in these words in our "boxes", and get the following setup for our search:

We use PubMed as the reference database for this search.

The first thing we do in PubMed is to look up all three main elements in PubMed’s controlled search vocabulary (MeSH – Medical Subject Headings – Medical search keywords/subject terms). This is a very important step in the process and helps us find the most important search keywords for our main elements.

Here exemplified with the first main element, Analgesics.

Then, you click on the hyperlink ‘Analgesics’ and are directed to the main page for the controlled search keyword you wish to use.

On this screen, there are several elements to be aware of. It is important to read the explanation of the controlled search keyword to make sure it coincides with your own understanding of what the terminology means. Then, you click on the button ‘Add to search builder’ to move the controlled search keyword into the PubMed search builder, and finally, you click on the button ‘Search PubMed’. You are then directed to PubMed’s search window for reference searches, which shows all the references that have been read and indexed with the MeSH term ‘Analgesics’.

Step 3: Build the first search

Now you have found the controlled search keyword for the first "box" (Analgesics) and transferred it from the MeSH dictionary to PubMed. You have now started the process of building your first literature search for this project. To carry out this literature search, repeat the process for the two remaining search keywords in the next two "boxes", ‘radiculopathy’ and ‘intervertebral disk displacement’. You have now transferred 3 controlled search keywords (MeSH terms) from the MeSH database to PubMed. You are now ready to combine these search keywords for your first search for this project.

We recommend that you always combine search keywords in ‘Advanced’ (see below). In ‘Advanced’, you have a full overview of all the search keywords you have used, and you can easily see how to combine them into a search.

When you now click on ‘Advanced’, the following screen appears:

Under ‘History and Search Details’ you find the entire search history for your controlled search keywords, and you are now ready to build your first search. You do this by clicking on the three dots you find under the heading Actions in ‘History and Search Details’ on the ‘Advanced’ page.

You now get three options: ‘Add query’ (create a search), ‘Delete’, and ‘Create alert’. You should now choose ‘Add query’.

You see now that your first controlled search keyword ‘Analgesics’, automatically places itself in the ‘Query box’.

Then, you click on the three dots under ‘Actions’, to the left of your next controlled search keyword.

Here you choose the Boolean operator AND, since we want to find literature that deals with all the search keywords in the same publication. Repeat this for the last controlled search keyword, and you now get the following screen:

By clicking on the ‘Search’ button to the right of this screen, you can choose to search directly in PubMed, or add the result of your first search directly to ‘History and Search Details’ (in practice, both happen). We choose to click on ‘Add to Search History’.

Line #4 under ‘History and Search Details’ shows that we have found 40 articles in PubMed that are all indexed with all of our controlled search keywords. You have now built and conducted your first literature search.

By clicking on the number 40 (hyperlink) under ‘Results’ on line #4, you are sent to PubMed’s search results page, and you can now see all the references found in your first literature search.

Step 4: Improve the search

The search we have now conducted is one where we have only used controlled search keywords for each main element from the project title. In an ideal world, this would be sufficient, but in the practical world, you can improve the search by also using some free search keywords in what we call a text word search. Indexing, or "tagging," of articles with controlled search keywords, such as MeSH terms, takes time. Therefore, there may be delays of several months, or longer. Additionally, those who index can make small errors. Therefore, it is always advisable to build a literature search that also includes one or more free search keywords, in addition to the controlled search keywords.

We start with our example:

Where can you find free search keywords for the three main elements?

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. The best way to do this is by starting to read articles and noting down the terminology researchers use to describe what you might be interested in.
Another very useful tool is to look up the individual main elements you have identified and translated to the controlled search keyword vocabulary. In PubMed, this is the MeSH database, which we have previously used.

If we look up ‘Analgesics’ in the MeSH database, we find something called ‘Entry Terms’:

Here you will find a range of synonymous text words, which can be used as free search keywords. Text word searches are done in selected search fields. The two most common search fields to search in PubMed are: ‘Title’ or ‘Title/Abstract’.

The procedure is based on the box system we have introduced. Here you enter the free search keywords (Entry Terms) you want to use, under each of the controlled search keywords.

For ‘Analgesics’, we choose to use ‘Analgesics’ which we truncate using the asterisk star * and Anodyne* for our text word search. We base our choice of free search keywords for our text word search on terminology that has been used in articles we have read so far, or found using ‘Entry Terms’ from the MeSH database, and we note this in our box model for the literature search as follows:

Here you see the beginning of a figure, which, when completed, can be published in the methods section of your bachelor’s thesis as a visual explanation of how you have built and conducted your literature search.

We have now found the free search keywords we want to use together with the controlled search keyword ‘Analgesics’ (MeSH term), and repeat the procedure for the controlled search keyword ‘Radiculopathy’.

From ‘Entry terms’ to ‘Radiculopathy’, we choose ‘Radiculopath*’ and ‘Nerve Root Disorder*’. Our box model for the literature search now looks like this:

We do the same for our last main element, ‘Intervertebral Disk Displacement’. From ‘Entry terms’ to ‘Intervertebral Disk Displacement’ we choose to use: ‘Disk Protrusion*’, ‘Disk Prolapse*’, and ‘Prolapsed Disc*’.

We have now completed our search setup for the literature search for our bachelor’s thesis. This figure is a publishable overview of the search setup for the literature search for your bachelor’s thesis:

What remains now is to conduct this search in PubMed. A good tip is to do this in a structured and clear manner. This means that you start with the keywords in the box to the left, and search each keyword individually. We start by retrieving ‘Analgesics’ from the controlled search vocabulary. Then we enter the free search keywords we have chosen from ‘Entry Terms’ to ‘Analgesics’. We do this in the window ‘Add Terms To Query Box’. Now we can choose which search field(s) we want to search in, by clicking on the drop-down menu to the left of the search keyword. For your bachelor’s thesis, it is more important to get precise hits, rather than building a sensitive search. We therefore recommend that you use the ‘Title’ field, for this search. If you get few results, you should rather use the ‘Title/Abstract’ field for your text word search. We choose the ‘Title’ field which gives us results in all references with the word ‘Analgesic*’ in the title:

Then we click on ‘ADD’, and place this search keyword in the ‘Query Box’. Then we click on ‘Add to History’.

We now see that the search for Analgesic* in the title field has given us 22,367 references with the word Analgesic* in the title.

We repeat this for the remaining free search keywords in our first box, and get a result that looks like this:

Notice that the search keyword Anodyne* only yields 44 references. This tells us that it is a synonym that is very rarely used.

We now combine these three search keyword searches from our first box with the Boolean operator OR and find that we have increased the number of references from 223,256 to 229,197. Thus, we have found quite a few more relevant references than we did by using only the controlled search keyword ‘Analgesics’.

We repeat this process for the remaining main elements, where we search up one search keyword from each box before combining them with OR. We then get a search setup that looks like this:

We can now make our final compilation of this search by clicking on the three dots under ‘Actions’, and choosing AND. The search then becomes: #4 AND #8 AND #13:

We see that we now find 44 references, which is 4 more than what we found by using only the controlled search keywords. You might think this was a lot of work for a few new references. However, the number of references you find with this methodology will vary greatly, depending on your project! This methodology provides you with a unique and structured literature search and will in most cases find very relevant references for use in your bachelor’s thesis. Additionally, it clearly shows that you have mastered the methodology underlying your literature study, allowing you to shine as a bachelor student when you formulate the methods section of your thesis. Good luck!

Step 5: Adapt the search to other databases

For most literature studies at the bachelor level, it will be sufficient to use one database. You may discuss with your supervisor if you need to conduct your literature search in multiple databases. PubMed, CINAHL, and PsycINFO are relevant databases to conduct your literature search in. The interfaces of these databases are different, but the search methodology explained above can be used in almost any reference database.


Example from CINAHL

In this example we use the following research question:

How can nurses contribute to reducing restraint use with people with dementia in nursing homes?

Step 1: From research question to searchable terms

Before you begin to identify search terms, it is important to break down your research question into its main elements. Ask yourself the following question: which concepts in the research question must be discussed in a source for that source to be relevant?

In this example, we identify the following main elements:

  • Nurses
  • Use of restraint
  • People with dementia
  • Nursing homes

To organize these elements and prepare to find search terms, we recommend setting the main elements as headings in separate "boxes". This gives you a good overview of the individual search terms and helps you conduct the search in a structured way.

Step 2: Find search terms for each main element

In this example, we will first search in CINAHL (Cumulative Index to Nursing and Allied Health Literature), one of the most important bibliographic databases in nursing. If you are unsure which bibliographic database is best suited to your research question, ask your advisor or contact the University Library.

We are now ready to find controlled vocabulary search terms that can be used in CINAHL. CINAHL has its own controlled vocabulary, called "CINAHL Headings". We recommend that you use controlled vocabulary search terms where possible to find the most relevant literature for your research question.

There are several methods to find controlled vocabulary search terms. In this example, we will use an automatic function in CINAHL that suggests relevant "CINAHL Headings".

In the CINAHL database, navigate to 'CINAHL Headings'.

We make sure that 'Relevancy ranked' is selected and type our first main element, Nurses, into the search box. Then, we click on the search icon.

In the search results, we find suggestions for relevant "CINAHL Headings":

If we click on 'Nurses,' we can see how this controlled vocbulary search term is defined in CINAHL. Always read the explanations of controlled vocabulary search terms to make sure that they correspond to your own understanding of what the terms mean.

We can also see where this controlled vocabulary search term is located within CINAHL's controlled vocabulary. Be sure to check whether there is a broader or narrower term in the hierarchy that might be more suitable for your research question.

To use a controlled vocabulary search term in your search, select the relevant search term. At bachelor level, we generally want to make sure that 'Explode' is checked where possible. This will automatically include all subordinate search terms in the search.

We then click 'Add to Search', followed by 'Search'.

On the search results page, we find a list of all sources indexed in CINAHL that are 'tagged' with the controlled vocabulary search term 'Nurses'.

We repeat this process for the other three main elements of our research question: Use of restraint, People with dementia, and Nursing homes. You may find more than one relevant controlled vocabulary search term for some main elements. In that case, make sure you search for one controlled vocabulary search term at a time.

We update our search setup as we go with the controlled vocabulary search terms we have found. MH stands for 'Main Heading' and is the code used in CINAHL for controlled vocabulary search terms (CINAHL Headings). The plus sign (+) indicates that we have selected 'Explode' when searching with a controlled vocabulary search term.

In this stage of the search, we have used the automatic feature in CINAHL to find at least one controlled vocabulary search term for each of the main elements of our research question, and searched with each term in the database.

Step 3: Build the first search

We are now ready to combine the controlled vocabulary search terms we have found to build our first full search for this project.

Search terms are combined in CINAHL in the 'Recent activity' window. This gives a complete overview of all the search terms we have used in the current session, and we can combine them easily to build a search.

The first thing we do is combine controlled vocabulary search terms that are in the same box using the combination word OR. In the 'Recent activity' window, we check the two controlled vocabulary search terms we have found for the main element People with dementia (S3 and S4). Make sure that the search box at the top is empty. After that we click on the combine icon and select 'Combine with OR'.

S4 OR S3 is added to the search box at the top of the page. Now we click on the search icon.

S7 will find sources that are indexed in CINAHL with either the controlled vocabulary search term 'Dementia+' or 'Dementia Patients'.

We do the same thing for the two controlled vocabulary search terms we have found for the main element Nursing homes (S5 and S6):

We are now ready to combine the four individual searches that correspond to our four main elements using the combination word AND. We check S1, S2, S7, and S8. After that we click on the combine icon, select 'Combine with AND,' and search.

On the search results page, we find the 17 sources in CINAHL that are indexed with at least one controlled vocabulary search term for each of our four main elements. We have now constructed and conducted an initial structured literature search.

Step 4: Improve the search

The search we carried out in the previous step only used controlled vocabulary search terms (in CINAHL, "CINAHL Headings"). When searching in a structured way, however, it is good practice to use one or more free-text terms for each main element, in addition to the controlled vocabulary search terms we have found. Free-text searches are carried out in selected search fields, such as the title field, abstract field, or both.

Where do we find free-text terms for our four main elements? There are several methods we can use. In this example, we will look more closely at the definitions in CINAHL for the controlled vocabulary search terms we identified, particularly under the heading 'Used for:'. We have a look for relevant synonyms, including singular and plural forms, and end up with the following relevant free-text terms:

  • nurses, nurse
  • physical restraint, physical restraints
  • dementia
  • nursing homes, nursing home

We record the free-text terms we have found and specify which fields in the reference information we will search in. Here, we choose to search in the Title field (TI), which gives us the following "box" setup:

Note that we have placed all search terms in quotation marks (""). We have also used the truncation symbol (*) with a couple of the free-text terms. By searching for TI ("physical restraint*"), we will simultaneously search for both 'physical restraint' and 'physical restraints'.

The search setup for our literature search is now complete. We can use this figure to document our search strategy.

What remains is to conduct this search in CINAHL. A useful tip here is to proceed in a structured, logical manner. This means starting with the search terms in the box on the left and searching with each term individually. We begin by retrieving "Nurses+" from the controlled vocabulary search vocabulary. This search gives us over 240,000 results.

Then, we enter the free-text term "nurse*". This can be done in 'Advanced Search'. We enter the free-text term "nurse*" in the search box. After that, we select the field 'Title - TI' from the dropdown menu on the right and click 'Search'.

Alternatively, we can use 'Basic Search' and search with TI "nurse*".

The search for "nurse*" in the title field gives us nearly 200,000 references.

The next step is to combine these two search terms in our first box by selecting S1 and S2 in 'Recent activity' and clicking on 'Combine with OR'.

When we click on the search icon, we see that we have increased the number of references to over 350,000. In this way, we have found quite a few more potentially relevant references than we did by using only the controlled vocabulary search term "Nurses+".

We repeat this process for the remaining elements, taking each box in turn and searching with each search term one at a time, before combining all the terms in the same box with OR. It is difficult to take screenshots of long searches on the 'Recent activity' page in CINAHL, but we can summarize the search setup so far with a table:

We can now finally put everything together by selecting the four OR searches we have carried out (S3, S6, S10, and S14), and clicking on 'Combine with AND':

S3 AND S6 AND S10 AND S14 is added to the search box at the top of the page. When we run this search, we see that we now find 19 references, which is 2 more than we found by using only the controlled vocabulary search terms.

We have now built a structured search in CINAHL that uses a combination of controlled vocabulary and free-text terms. This methodology will, in most cases, find highly relevant references for use in a bachelor thesis.

Step 5: Adapt the search to other databases

At bachelor level, one bibliographic database will often be enough. You can discuss the need to conduct your search in multiple databases with your advisor.

PubMed and Web of Science are among the other relevant databases for this research question. The controlled vocabulary used in PubMed is called Medical Subject Headings (MeSH). When "translating" a search strategy from CINAHL to PubMed, an important step is to check that there are MeSH terms that correspond to the controlled vocabulary search terms we used in CINAHL (CINAHL Headings). We can use the same free-text terms in both databases.

Web of Science lacks a controlled vocabulary. In this database, then, we can only use free-text terms.