code: public static ArrayList<String> addInteraction(String variable1, String variable2, ArrayList<SurvivalInfo> survivalInfoList) {
  		ArrayList<String> variables = new ArrayList<String>();
  		variables.add(variable1);
  		variables.add(variable2);
  		variables.add(variable1 + ":" + variable2);
  		for (SurvivalInfo si : survivalInfoList) {
  			Double value1 = si.getVariable(variable1);
  			Double value2 = si.getVariable(variable2);
  			Double value3 = value1 * value2;
  			si.addContinuousVariable(variable1 + ":" + variable2, value3);
  		}
  		return variables;
  	}
docs: To test for interactions use two variables and create a third variable where the two are multiplied together.
  @param variable1
  @param variable2
  @param survivalInfoList
  @return