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Inter-item Reliability

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Inter-item Reliability

With inter-item reliability or consistency we are trying to determine the degree to which responses to the items follow consistent patterns.

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For exampleLet’s imagine that two items are designed to assess how “teachable” someone is. Item 1 is a difficult item for someone to agree with. So on a scale of 1 – 5 with 1 being “Strongly Disagree” and 5 being “Strongly Agree” here is what the response pattern might look like:

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For exampleLet’s imagine that two items are designed to assess how “teachable” someone is. Item 1 is a difficult item for someone to agree with. So on a scale of 1 – 5 with 1 being “Strongly Disagree” and 5 being “Strongly Agree” here is what the response pattern might look like:

Respondent

Item 1

Bob 1

Carla 2

Don 1

Elsa 3

Fiona 1

Gordon 2

Etc.

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For exampleNow take a look at item 2. It is much easier to agree with:

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Fiona 1 4

Gordon 2 5

Etc.

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Fiona 1 4

Gordon 2 5

Etc.

Notice the

consistency in the

item response patterns

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Fiona 1 4

Gordon 2 5

Etc.

Bob & Fiona are less likely to agree

with these items

targeting teachablen

ess.

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Fiona 1 4

Gordon 2 5

Etc.

But even though

they are both less likely to

agree they consistentl

y agree more with

item 2 than 1.

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Fiona 1 4

Gordon 2 5

Etc.

This shows a

consistent pattern across items.

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Bob 1 4

Gordon 2 5

Etc.

Notice that this pattern

holds across all

of the other

respondents. Item 2 is easier to

agree with than Item 1

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Bob 1 4

Gordon 2 5

Etc.

Notice also that Elsa is the most agreeable

across both items.

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Bob 1 4

Gordon 2 5

Etc.

And Bob & Fiona are consistently the least agreeable

across both items.

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Bob 1 4

Gordon 2 5

Etc.

The item responses to this instrument, therefore,

demonstrate high inter-item reliability or consistency

across persons with different levels of the trait.

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For example

Respondent

Item 1

Item 2

Bob 1 4

Carla 2 5

Don 1 4

Elsa 3 6

Bob 1 4

Gordon 2 5

Etc.

This type of reliability is measured principally by a statistic called Cronbach's

Alpha and ranges traditionally from 0.00 (no

consistency) to +1.00 (completely consistent).