Walker Downey & Associates, Inc.

Lesion Grading or Semi-Quantitative Histopathology

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There are two types of pathology data in nonclinical toxicology studies. These include qualitative variables or nonnumeric variables that can't be measured, and quantitative variables or numerical variables that can be measured. Examples of qualitative variables include gender, animal species/strain, and supplier. Examples of quantitative variables include most clinical pathology parameters (i.e. hematology and clinical chemistry values), and body and organ weights. Note that quantitative variables are either discrete (which can assume only certain values, and there are usually "gaps" between the values, such as the specific grading of histopathological lesions) or continuous (which can assume any value within a specific range, such as animal age, body weight, or bilirubin value.)

In statistical terms there are four (4) types of quantitative data. These include:

• Nominal Data: The weakest data measurement. Numbers are used to represent an item or characteristic. Examples include: the presence or absence of a specific change being indicated as 0=absent, 1=present; or gender indicated as male=1 and female=2. Note that such data should not be treated as numerical, since relative size has no meaning.
   
• Ordinal or Rank Data: Numbers are used to rank. An example is the severity grading of a lesion as minimal=1, mild=2, severe=3; or the grading of animal general health as excellent, good, fair and poor (with associated arbitrary numbers). Simple arithmetic operations are not meaningfully applied to ordinal data. The main difference between ordinal data and nominal data is that ordinal data contain both an equality (=) and a greater-than (>) relationship, whereas the nominal data contain only an equality (=) relationship.
   
• Interval Data: If we have data with ordinal properties (> and =) and can also measure the distance between two data items, we have an interval measurement. Interval data are preferred over ordinal data because, with them, decision makers can precisely determine the difference between two observations, i.e., distances between numbers can be measured.
   
• Ratio Data: Is the highest level of measurement and allows for all basic arithmetic operations, including division and multiplication. Data measured on a ratio scale have a fixed or nonarbitrary zero point.

Walker Downey & Associates has extensive experience in the analysis of semi-quantitative histopathological scoring of tissue lesions. Oftentimes our contemporary approach will employ evaluations “with and without sexes combined”, and interval data when available. Where necessary we can apply transformations to normalize the data and increase the power of the analysis to the point where parametric tests are applicable. Please contact us at info@walkerdowney.com to discuss how we may help to improve your semi-quantitative analyses.