Inventory vs questionnaire, in research, is not a choice between two different tools: an inventory is a kind of questionnaire. It is a published, tested set of items that measures a trait, interest, skill or symptom with a fixed scoring rule. A questionnaire is any set of questions written to collect facts, behaviour or opinions.
A research methods textbook gives the clearest example: the Beck Depression Inventory is "a 21-item self-report questionnaire", scored by adding its 21 ratings. The textbook is Research Methods in Psychology by Price, Jhangiani and Chiang (chapter 5). So the real decision is whether to use an existing inventory or write your own questions.
If you came for inventory in the business sense, the goods a shop holds to sell, read inventory vs supplies instead.
What is an inventory in research?
An inventory is an existing instrument that takes stock of one area of a person, item by item. Its wording, response options and scoring stay fixed, and its authors have published evidence that it is reliable and valid.
The common kinds, with an example of each:
- Personality inventories. The Ten-Item Personality Inventory measures the five broad personality dimensions with just 10 items.
- Symptom inventories. The Beck Depression Inventory asks how much a person has felt each of 21 symptoms over the past two weeks.
- Interest inventories. They show what a person likes doing, and career guidance uses them to match people with work.
- Skills inventories. People rate their own skills, one by one. The result is what they believe they can do, so add a test when real ability matters.
- Concept inventories. Multiple-choice tests of what students understand in a subject. They are the exception covered below.
Many research inventories are free to use with a citation. Many clinical ones are owned by a publisher and must be bought, including the Beck Depression Inventory and the MMPI (the same textbook, chapter 5).
What is a questionnaire?
A questionnaire is the general instrument: written questions a person answers on paper, by phone or online. A researcher writes one for each study, so it fits the research question exactly. It starts with no evidence that its items measure anything well.
Its items are open-ended, where people answer in their own words, or closed-ended, where they pick from options such as a rating scale. The textbook's BRUSO rule says each item should be brief, relevant, unambiguous, specific and objective (chapter 9). Try the items on a few people first and ask how they understood each one.
Inventory vs questionnaire side by side
| Point | Inventory | Questionnaire |
|---|---|---|
| What it measures | A construct: a trait, interest, symptom, skill or concept | Facts, behaviour, opinions, or a construct with no existing measure |
| Who writes it | Its authors, once; you use it as published | You, for this study |
| Items | Fixed, such as 10 in the TIPI and 21 in the Beck inventory | As many as your question needs |
| Scoring | A fixed rule, often a sum per scale | Item by item, or a scale you build |
| Evidence of quality | Published reliability and validity | None until you pilot and test it |
| Cost | Free with a citation, or bought from a publisher | Your time |
| Example | A depression or personality inventory | A survey of how market traders keep records |
Which one should you use?
Use an existing inventory when one already measures your construct, and write questionnaire items for everything else. An existing measure saves time, brings evidence that it works and lets you compare your results with earlier studies (chapter 5).
- A trait, symptom, interest or attitude others have measured: use a published inventory, cite it and report its reliability in your own sample.
- Facts about your participants: age, trade, years in business or what they do. Write closed questionnaire items.
- Opinions on something only your study covers: write questionnaire items, closed where you can foresee the answers and open where you can't.
- Most studies need both: a short questionnaire for background, then one or more inventories.
Check reliability in your own data. A Cronbach's alpha of +.80 or more is generally taken as good internal consistency (chapter 5).
Translating an inventory into Swahili, Luganda, Hausa or any other language changes the instrument. The published evidence no longer covers it, so translate, back-translate, pilot and check reliability again.
The International Test Commission publishes guidelines for translating and adapting tests. Rewording or dropping items changes an inventory in the same way.
Does an inventory have right answers?
Usually not, and that is where the name comes from. Course notes at Old Dominion University say "inventory" is used instead of "test" when the instrument has no correct or incorrect answers (OTED 635, lesson 6).
Concept inventories break that rule. The Force Concept Inventory, first published in 1992, is a 30-question multiple-choice test of basic physics with an answer key (PhysPort). The dependable difference is not right answers: it is a fixed, tested instrument versus questions written for one study.
Frequently asked questions
Is an inventory a test?
A personality or interest inventory is not, because no answer is right or wrong. A concept inventory is. Both are standardised instruments with fixed items and scoring.
Is a survey the same as a questionnaire?
No. A survey is the whole study: choosing people and collecting their answers. The questionnaire is the instrument the survey uses.
Can I shorten a published inventory?
Only with care, because a shortened inventory loses the evidence behind the full one. Use a published short form, such as the TIPI for the five personality dimensions, rather than cutting items yourself.
Is a Likert scale an inventory?
No. A Likert scale is one format: agreement with several statements on a 5-point scale, summed into one score. Many inventories use it, and so do many questionnaires.
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