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# Indefinite Article (a/an)

Learn how to use the indefinite article *a/an*, and see some examples in a text about linear regression.

## Form

There are two indefinite articles: a and an. An is used before words that start with a vowel sound, a, e, i, o, u. For example: an organisation, an effect, an accurate analysis.

A is used before words that do not start with a vowel sound. For example: a graph, a relationship, a model, a database.

Be careful, sometimes a word starts with a vowel, but it is not said with a vowel sound. This is common with words that start with the letter u but which have the sound y. For example: university, unit. For these words you need to use a: a university, a unit.

### Use

The indefinite article, a or an, is used before singular nouns. It has a number of uses:

1. For the first time

A is used when you refer to something for the first time, so people do not know what it is.

Here’s an example:

It is sometimes called a trendline or a line of best fit.
In the example, this is the first time the text has said trendline and line of best fit, so they use a not the. If they referred to them again later in the text, they would write the trendline and the line of best fit.
2. For a non-specific noun
Use a or an to refer to a noun that is part of a group, but which you have not defined or identified. In other words, you are not referring to a specific example.
Here’s an example:
In a graph, the independent variable is the x-axis (the horizontal line) and the dependent variable is the y-axis (the vertical line). You can see these labelled in the graph below.
At the beginning of the sentence, a graph is used because it is not referring to a specific graph. At the end of the sentence the graph is used because it is referring to the specific graph shown below the text.
3. To mean one
Use a or an to mean one of something.
Here’s an example:
Linear regression is a model used in data science.

We use a here because there is more than one model in data science. If linear regression was the ONLY model, you could say linear regression is the model used in data science.

### Task 1

Here are some more examples from the reading passage. Can you work out why the indefinite article, a or an, has been used in each example?

Example Reason used
For example, linear regression might be used to answer a question like this …
Therefore, in the example, productivity changes as a result of how many hours of sleep a person gets.
In a linear relationship, there is also “negative correlation” and “no correlation” as you can see in the graphs below.

You can download the answers here: Task 1 ANSWERS

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