How to Set Up BMI Calculation in Chloe

Overview

This article outlines the protocol for setting up Body Mass Index (BMI) calculations within a Chloe study. BMI is a derived value calculated using participant-entered height and weight. Chloe supports real-time BMI calculation through form logic using JavaScript syntax and dynamic field assignments.

Step-by-Step Setup

1. Height Field Setup

Set-up a height field in one section of the form that you would like to contain the BMI calculation.

• Recommended Format: Use a dropdown menu that displays height in feet and inches for participant clarity.

• System Functionality: Each selection corresponds to a pre-converted meter value that is used for BMI calculation.

• Reference Table: See the Height Dropdown Mapping section below for the list of saved values and visible options that can be copied into the Free Form section of Chloe.

• Schema Field: Name your schema field Height and code it as a numeric value with precision 4 and scale 2.

2. Weight Field Setup

• Recommended Format: Use a numerical input field where participants enter their weight in pounds.

• Schema Field: Name your schema field Weight and code it as an integer value.

3. Assign Action After Entry

• Add an "Assign" action to the weight field so that after participants complete both the height and weight fields their BMI is calculated.

• This action will trigger the calculation of the BMI value and store the result in a designated field (BMI).

4. Variable Specification

• Choose or create a data variable (e.g., bmi) to store the calculated BMI value.

5. BMI Calculation Setup

• In the Assign action, switch to "Compute a value".

• Use the following JavaScript formula to calculate BMI:

(0.45359237 * weight) / (height ** 2)

💡 Note:

• weight should be in pounds.

• height should be in meters (as selected via the dropdown).

• The formula converts pounds to kilograms and uses meters for height to calculate BMI in kg/m².

Height Dropdown Mapping

Use this table to configure the height dropdown options in your form:

Key:

saved meter value|participant representation

Copy below

1.22|4 ft.
1.24|4 ft. 1 in.
1.27|4 ft. 2 in.
1.3|4 ft. 3 in.
1.32|4 ft. 4 in.
1.35|4 ft. 5 in.
1.37|4 ft. 6 in.
1.4|4 ft. 7 in.
1.42|4 ft. 8 in.
1.45|4 ft. 9 in.
1.47|4 ft. 10 in.
1.5|4 ft. 11 in.
1.52|5 ft.
1.55|5 ft. 1 in.
1.57|5 ft. 2 in.
1.6|5 ft. 3 in.
1.63|5 ft. 4 in.
1.65|5 ft. 5 in.
1.68|5 ft. 6 in.
1.7|5 ft. 7 in.
1.73|5 ft. 8 in.
1.75|5 ft. 9 in.
1.78|5 ft. 10 in.
1.8|5 ft. 11 in.
1.83|6 ft.
1.85|6 ft. 1 in.
1.88|6 ft. 2 in.
1.91|6 ft. 3 in.
1.93|6 ft. 4 in.
1.96|6 ft. 5 in.
1.98|6 ft. 6 in.
2.01|6 ft. 7 in.
2.03|6 ft. 8 in.
2.06|6 ft. 9 in.
2.08|6 ft. 10 in.
2.11|6 ft. 11 in.
2.13|7 ft.
2.16|7 ft. 1 in.
2.18|7 ft. 2 in.
2.21|7 ft. 3 in.
2.24|7 ft. 4 in.
2.26|7 ft. 5 in.
2.29|7 ft. 6 in.
2.31|7 ft. 7 in.
2.34|7 ft. 8 in.
2.36|7 ft. 9 in.
2.39|7 ft. 10 in.
2.41|7 ft. 11 in.
2.44|8 ft.