# Example of a multiple variable expression using logical operators

This example illustrates a method for isolating biomass in an echogram, according to a classification rule or rules. A rule or rules can be particularly useful with Sv variables that have been recorded in parallel at different frequencies. Depending on the parameters of the classification rule(s), it may be possible to separate biomass due to fish species or biomass due to other sources.

Echoview's Frequency response graph is one tool that could identify parameters for a classification rule.

The screenshot (above) shows part of a multiple frequency virtual variable analysis to identify Pollock. The set of green virtual variables can be replaced by the highlighted (yellow) Autopollock virtual variable. Autopollock uses the Formula operator's logical expression and multiple input capabilities as displayed in the formula fragments below.

Multiple input capability:

Part of a classification rule.

Logical expression capability:

**Note:** The formula operator does not account for data type when it evaluates operations on data values. As a consequence, you need to be aware of your data types and design your formulae with respect to linear domain and dB domain calculations accordingly. Refer to Input operands, pings, samples and data values for more information.

#### See also

About the Formula operator

Creating a formula

Formula operator