Answer to Question 1
Multi-item instruments for measuring a construct are either called index measures or composite measures. An index measure assigns a value based on how characteristic an observation is of the thing being measured. Indexes often are formed by putting several variables together. For example, a social class index is based on three weighted variables: income, occupation, and education. With an index, the different attributes may not be strongly correlated with each other. Composite measures also assign a value based on a mathematical derivation of multiple variables. For most practical applications, composite measures and indexes are computed in the same way. However, composite measures are distinguished from index measures in that the composite's indicators should be both theoretically and statistically related to each other.
Answer to Question 2
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Measures of ocean chlorophyll-a concentrations from part of Atlantic Canada in (a) January 2006, and
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