WP4 input to risk calculation formula

This line says:

/**
 * Scoring strategy that implements the WP4 formula
 * risk = - SUM_i=1_to_I((5*delta_t(i, i - 1))/min((RSSI(i) + RSSI(i - 1)) / 2 + alpha, -5) * 60
 * where alpha = - RSSI_1m - 5
 * For contacts with a single message, cap estimated delta to 120 seconds
 */

What is the WP4 formula? Where does it come from?

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