sensor data question

Using Arduino Programming Questions
13 
  prevSensorValue = analogRead(pingPin);
int currSensorValue = analogRead(pingPin);
   if((abs(currSensorValue - prevSensorValue)) > bigChange)

No! currSensorValue is for this pass through loop. prevSensorValue is from last time, NOT this time.

   int currSensorValue = analogRead(pingPin);
   if((abs(currSensorValue - prevSensorValue)) > bigChange)
   {
      // a change occurred
      then = millis();

     digitalWrite(led13, HIGH);
     // Do something with the sensor data
   }
   prevSensorValue = currSensorValue;