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Employee
Employee

Scales of Measurement

As you load data into QlikView or Qlik Sense, it is useful to ask the question: What type of field is this? Which properties does it have?  Different categories of fields have different properties:


The first category is Nominals. These are fields with discrete, qualitative values. There is no inherent quantitative difference between different values of a field. Examples: Product, Customer, Color, Gender, etc.

Pyramid2.png

The second category is Ordinals. These fields also have discrete values but the fields differ from the Nominals in that they have an intrinsic order. Examples:

  • low, medium, high
  • tiny, small, medium, large, huge
  • unsatisfied, neutral, satisfied

The ordinals can sometimes be numeric but should still not be thought of as numeric, since the distance between one value and the next may differ from case to case. This means that you cannot calculate an average – but you can calculate a median.

The next category is numeric: Intervals. These can be discrete or continuous. Examples: Date, Time, Longitude, Latitude, Temperature (°C or °F). What makes them different from Ordinals is that the difference between two values is well-defined: The difference between a temperature of 0 degrees and 10 degrees is the same as between 70 degrees and 80 degrees. Such fields always describe a position in time, in space or in some other dimension. I find the term “Interval” to be confusing so I think of them as Coordinates instead.

Intervals are not additive, so you cannot sum them. However, you can calculate a difference between two values and use this value for further calculations.

The last category is Ratios. The Ratio category is the most informative one. It has all properties of the Interval category, with the additional property that zero is special: it indicates the absence of the quantity. Examples: Sales amount, Weight, Length, Order quantity, etc. Further, they are often additive. Since I think the term “Ratio” is misleading, I think of them as Amounts instead.

The above taxonomy was created by the psychologist S. S. Stevens in the early 1940s and is normally referred to as Scales of Measurement. Although it has been criticized from a scientific perspective, I find the classification useful since a number of rules of thumb for visualizations can be tied to this model. For instance:

  • Nominals should be sorted by a measure or alphabetically. Other categories should be sorted according to the intrinsic sort order.
  • Nominals should never be used as first dimension in a Line chart, since this chart type implies an intrinsic sort order.
  • Pie charts should not be used, unless the dimension is a Nominal.
  • Scatter charts are best if they have a Nominal or Ordinal as dimension.
  • Continuous Intervals and Ratios should normally not be used as dimensions. Use Round() or Class() to make them discrete.
  • Ordinals should not be used to calculate an average.
  • Intervals should not be used to calculate a sum.
  • The axis of a Ratio should start at zero and not be broken.

Table2.png

I am sure that some of you can find exceptions to the above “rules”, but as I said – they are only rules of thumb.

The bottom line is that you should think about the field categorization before you create your visualizations. Thank you Michael B for inspiration and discussions.

HIC

Further reading related to this topic:

Additive and Non-Additive Numbers

14 Comments
Not applicable

Henric,

This is a great article, and explains some concepts that I probably knew, but gives me a way I can clearly communicate to my staff.  Thanks very much for this.

However whilst I understand the classifications, I don't understand what the first chart is communicating?  Do you think there is an order here? are there more ratios to nominals?  are the colours trying to convey importance?  Am I reading too much into an illustration?

Thanks

Richard

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Employee
Employee

It is just trying to convey that there is an order of the categories:

  • Nominals is the most basic category with very few properties.
  • Ordinals have the properties of Nominals, but with the added property of an intrinsic order.
  • Intervals have the properties of Ordinals, but with the added property of numeric interpretation.
  • Ratios have the properties of Intervals, but with the added property of a natural zero.

HIC

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Not applicable

Henric,

Thank you for this article, it was really useful for me.

Excellent article

Ryuma

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kalyandg
Contributor II

Dear HIC,

Thank you very much for this article.

Best Regards,

Kalyan

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Not applicable

Great Post..

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Not applicable

Very useful

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MCampestrini
Valued Contributor

Dear HIC

Another great post, as usual.

Thanks.

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Good post Thank you.

Richard Garforth It's great to see how "visually literate" we have all become. Perhaps a scatter chart communicates it better

2014-09-03_13-52-18.png

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perumal_41
Valued Contributor II

Nice post.

periodic line chart  calculation will come under Intervals or Ratios?




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dvqlikview
Honored Contributor II

Another very informative post! Thank you HIC.

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marmentrout5
Contributor

Succinct information.  Thank you for sharing! @HenricIC

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neetha_p
Honored Contributor

Informative and Very Helpful post! Thank You Henric

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andrewpettit
Contributor

I think for a line chart showing values over time, the date dimension would be considered an interval, the field you are using in your expression would be a ratio ( assuming the most basic of expressions i.e. sum(sales) ), and if you have multiple lines the dimension driving that could be a nominal, ordinal or interval... example would be: sales, by year by month where each line might represent a different year and the points on the lines would be the sum of sales for the given month within each year. In this scenario our line chart would consist of two intervals and a ratio.

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durgesh22
New Contributor

Henric.

Thanks for the article.

.

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