# Index Value Including Ra Value Calculation

This is the process used by the system to calculate a condition category index value with Ra values. Condition Categories with Ra values are Background Ra and Legend Ra.

This calculation applies only to Signs.

1. Determine the prediction group of the sign.
Use the current Prediction Group value. If none exists, run the calculation to determine Prediction Group. If the Prediction Group cannot be calculated, the index is set to null.
2. Analyze the curve for this record’s prediction group and condition category, either Legend Ra or Background Ra.
If the curve has no points, the condition category index is set to null and the calculation is complete.
3. Use the entered Ra value to find the position on the curve and determine the corresponding index.
• If the reading is a defined point on the curve the defined corresponding index is used.
• Determine the two defined Ra points and surround the value you are looking for. One before and one after.
• If your point is before the earliest point the index that corresponds with year 0 is used. This index value is used regardless of the Ra value that corresponds to year 0, including if the Ra is Null at Year 0.
• If the point is after the last point, the last two points on the curve are used as the points.
• Points that have a null index or ratting are not included in the curve.
4. Use the following formula to determine the Index value that corresponds to the given Ra value.
• Use the formula: Point1.Index + (Point2.Index-Point1.Index)/(Point2.RA-Point1.RA)*(GivenRA-Point1.Ra) where:
• Point 1 is the previous point (to the left of your point)
• Point 2 is the after point (to the right of your point)
• Given Ra is the user-specified Ra value
• If the calculated index is less than zero then the index is set to zero