**Core Concept**
Ordinal scales are a type of measurement used in clinical research, where variables are categorized into distinct levels or orders, such as mild, moderate, and severe. This type of measurement is often used for subjective or descriptive data that cannot be quantified precisely.

**Why the Correct Answer is Right**
The correct answer, **C. Ordinal analysis**, is used to analyze data that has been categorized into distinct levels. This type of analysis is particularly useful for ordinal data, which cannot be ranked or ordered in a meaningful way. In the context of clinical research, ordinal analysis is often used to analyze data from surveys, questionnaires, or rating scales.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because **Nominal analysis** is used for data that has no inherent order or ranking, such as categorical data. While ordinal data can be categorized, nominal data cannot be ranked in a meaningful way.
**Option B:** This is incorrect because **Interval analysis** is used for data that has a true zero point and equal intervals between consecutive values, such as temperature or blood pressure. Ordinal data does not have these properties.
**Option D:** This is incorrect because **Ratio analysis** is used for data that has a true zero point, equal intervals between consecutive values, and can be meaningfully ranked, such as weight or height. While ordinal data can be ranked, it does not have a true zero point and equal intervals.

**Clinical Pearl / High-Yield Fact**
When analyzing data with ordinal scales, it's essential to note that the intervals between consecutive levels are not necessarily equal, and the data should be handled with caution to avoid misinterpretation. Additionally, ordinal data should not be treated as if it were interval or ratio data.

**Correct Answer:** C. Ordinal analysis is used to analyze data that has been categorized into distinct levels.