# Interval Scale

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Planning Values (PVs) are used to assess the relative degree of acceptance (suitability, sensitivity, vulnerability, risk, capacity, desirability, potential, etc.) associated with features on a map. For example, one might want to assess the development potential of an area based on the amount of slope.

The degree of acceptance, which is typically a subjective assessment, can be measured (to a limited degree) on this 9-point interval scale. The measurement is limited, with respect to its accuracy, by the expertise of the individual or individuals making the assessment.

The main body of the scale (the 9-points) is based on the theory of psychometrics. Psychometrics is the study of how to use objective tools and procedures (mathematics) for measuring subjective judgments and assessments (opinions).  See Psychometric Theory, by Jum Nunnally, published by McGraw Hill, 1957.

The scale enables individuals to first identify which features are rated as high (H), which are rated as medium (M), and which are rated as low (L). That individual, or group, can then look at all of the features rated high (H) and determine which ones are a bit higher or lower than the others by either assigning a plus (+) or minus (-) to the highs (the Hs) or leaving them as simply H without the added plus (+) or minus (-).

Following this thinking for each of the three value categories (the highs, mediums and lows) yields a 9-point interval scale going from very low (L-) to very high (H+). This scale is represented numerically as going from 1 to 9.

It is important to note that this is an interval scale and not a ratio scale. As such, following the rules of mathematics, the numbers on this scale cannot be used as multipliers, as would be the case when attempting to use this scale to measure the degree of influence assigned to map layers in a weighted overlay. In this case, the influence factors should be measured as a percentage where the total influence, when considering all of the factors, equals 100% (a ratio scale).

Three other values, or assessment indicators, have been added to the scale:

Restricted (R) – is used when a feature class is determined to be restrictive with respect to a specific measure of importance (such as, development suitability) regardless of what other conditions may exist. For example, an area of land might be thought to be suitable for development because it has great views, is relatively flat, and is close to existing roads. However, because it is also a wet-land it cannot be developed. In this case, the restricted value (R) would be assigned to the feature class “wet-lands” which would in turn tag the wet-lands feature class as being unsuitable for development.

Undetermined (U) – is used when data deemed to be important to the decision is not available. That is, when one of the feature classes on the map represents “no data”. At this point, the individual or group making the assessment can decide to proceed with the assessment of the other features as if the missing data is not important, most likely by assigning a medium (M) value to the “no data” category, or they can choose to delay the assessment process and wait until the missing data is available.

No Comment (N) – is used when a group of people are making an assessment, perhaps using the Delphi Process, and when one or more members of the group wants to defer their individual assessment to others in the group, as might be the case when an individual feels they lack the expertise to make the assessment and wishes to defer to those who do.

This scale is often used on land-based planning projects and is embedded as one of the value scales available in most GIS computer programs.