We checked it against the future.
We hid every result after 2015, ran the warning on what was left, then compared it with what 1,376 wells actually did from 2020 on. Our rule's median error was smaller than carrying the last value forward, and it caught 30.6% of wells that crossed 20 mg/L.
- Trend fit on results through 2015
- Gap
- Checked against 2020 on
Crossings caught
Wells at or under 20 mg/L before 2016 that went over it from 2020 on.
111 missed 49 caught, 30.6%
- 1,376
- wells tested
- 15
- false alarms
- 4.6
- median error, our rule (mg/L)
- 5.1
- median error, repeating the last value (mg/L)
Terms
- Last value
- the well's last sodium sample before 2016, carried forward as the guess for its samples from 2020 on
- Our rule
- the well's sodium trend before 2016, projected forward to its samples from 2020 on
- Crossed
- a well at or under 20 mg/L at its last sample before 2016 whose average sodium from 2020 on is over 20 mg/L
- Caught
- a crossed well that our rule projected over 20 mg/L
- False alarm
- a well at or under 20 mg/L at its last sample before 2016 that our rule projected over 20 mg/L but whose average sodium from 2020 on stayed at or under 20 mg/L