Predicting and Preventing Deaths in the ICU: Designing and Analyzing an AI System

Overview

In this assignment, students build a system for predicting patient deaths in the Intensive Care Unit (ICU) using the large PhysioNet Computing in Cardiology Challenge 2012 dataset. For each patient in the dataset, demographic variables and time series of physiological variables were collected during their stay in the ICU. Students model the impact of the system's flagging higher-risk patients for intervention on patients' survival in the ICU. The assignment illustrates the process of building an AI system and assessing its potential impact when only observational data is available.

Clinical risk prediction from physiological measurements, clinical notes, and demographic data using machine learning has seen advances in recent years. See Johnson et al. (2016) for a review of work on risk prediction for critical care and a popular press article (2019) about Google's efforts in the area.
Students generate a graph that shows the expected number of deaths in the validation set for different classifier thresholds for a scenario where risk prediction is used to prioritize charts, assuming the simulation model assumptions hold, and use the threshold that produces the smallest number of deaths in the final analysis that uses the test set.

Evaluating the impact of a hypothetical intervention precisely using only observational data is not possible. However, we believe it is still important, both pedagogically and practically, to try to assess the impact of a new AI system. Students use simulation to model a setup where doctors have a limited amount of time to review a large number of patient charts, and the system helps prioritize patients' charts. Students estimate the percentage of patients who could be saved in the test set assuming the simulation model assumptions hold. Students like seeing a concrete estimate for the effectiveness of a system that they made.

We used the assignment in an Introduction to Data Science class for non-majors. Many of the students in our course do not have prior programming experience. The assignment can also be used as a first assignment in an AI class that introduces machine learning.

Introduction to Data Science classes with minimal or no pre-requisites, and taught in R, are becoming commonplace. They often involve predictive modelling. That means that students outside of computer science are now exposed to AI/machine learning early in their academic careers. In this assignment, we demonstrate how these students can be engaged. Students write interesting functions. We are also happy with the striking visualization of the effect of using the system could have that the students produce.

Since part of our goal is to teach programming, we restrict ourselves to basic subsets or R/tidyverse and Python/pandas/scikit-learn. Our solution presumes that students will use the basic features to implement much of the solution rather than look for readymade functions that will solve parts of the assignment for them.

The assignment was designed for R/tidyverse, but we provide a translation to Python/pandas/scikit-learn. The assignment assumes students have already seen logistic regression in a predictive modelling setting earlier in the course, and that students are familiar with performance measures such as the false positive rate and the false negative rate from lecture. The assignment assumes students may be somewhat new to programming.

Meta Information

Summary

Students fit a logistic regression classifier to predict patient death in the ICU from physiological measurements and demographic information. Students use a simulation model of a setup where doctors have a limited amount of time to review a large number of patient charts, and the system helps prioritize the charts. Students estimate the percentage of patients who could be saved in the test set assuming the simulation model assumptions hold.

Topics

Logistic regression, parameter tuning, simulation, ROC curves, classifier performance measures

Audience

Students in introductory data science and AI classes.

Difficulty

This is a fairly difficult assignment for students in an Introduction to Data Science class that does not assume CS1. It could be used as a first assignment in a more advanced class.

Strengths
  • Relevance. Patient risk prediction systems are starting to be used in hospital settings. Students (and particularly pre-meds) enjoy working on a problem with real-world implications in healthcare.
  • Design process. Unlike in many introductory assignments, students do not simply fit a classifier to the training set and then evaluate it on the test set. Students need to explore a way to actually use the classifier: the correct classification rate on the test set does not say much by itself. Students see a larger part of the design process than they usually would.
  • Working with real data. Students work with a real large-scale dataset. Students interested in medicine were interested to find, for example, that higher body temperature is associated with slightly higher survival rates. (A finding that appears elsewhere in the literature as well.)
  • Teaching modelling and programming for data science in context. Our goal is to teach students programming and modelling skills while they work on a data science program they preceive as relevant. Students write interesting functions, and the model we use for simulating doctors' decisions in the ICU produces an interesting and striking visualization of the effect of changing the parameters of the model.
Weaknesses
  • Necessary simulation model assumptions. It is impossible to precisely evaluate the effect of intervention using a risk-prediction system without running an experiment. It is important to emphasize to students that they are using strong model assumptions.
  • R vs. Python. The assignment works well for beginner students in R/tidyverse. We translated the assignment to Python/pandas/scikit-learn, but the assignment likely works a little better in R/tidyverse for beginners. For example, vectorized operations like v1 & v2 in R are taught early, but the NumPy equivalent np.logical_and(v1, v2) seems to be a little less beginner-friendly, though objectively speaking the difference is minimal.
Dependencies R + tidyverse OR Python + pandas + scikit-learn, logistic regression for classification, classifier performance measures: false positives/false negatives/true positives/true negatives/correct classification rate.
Variants
  • The fact that the dataset is unbalanced results in the awkward fact that the classifier cannot beat the base rate. One possible remedy is to subsample the training set to make it balanced (which would reduce the base rate to 50%, and have the classifier easily beat the base rate). We chose not to do that for the main variant of the assignment for simplicity and because we'd rather not throw away data, but we actually assigned a version where we asked the students to make the training set balanced. It makes for a nice programming exercise.
  • Our class covers statistical analysis as well as predictive modelling. In the version of the assignment we used in class, we asked students to test a hypothesis about the data that we gave them, as well as to come up with and test their own hypothesis. Details are available upong request.
  • We provide students with code to read in the data, and only ask them to explain the code. It is possible to ask students to write their own code to read in the data, though probably not in a class like ours, which had no programming pre-requisites.
  • Variable selection on this dataset is not that interesting (at least when logistic regression is used as the classifier), so we omitted it from the main variant of the assignment. However, we actually asked students to do variable selection using the validation set in the variant we assigned.
  • It is possible to use classifiers other than logistic regression (e.g., decision trees), and to use computed features to improve the performance of the classifier.
  • The assignment is designed for students to make use of only the basic functionality of R and Python. However, for example, in Python, students could be made to use the features of scikit-learn to perform tasks such as generating the ROC curve.
  • Instructors may want to incorporate an analysis of the system for fairness, following e.g. Mitchell et al. (2018)

R Handout

Main project (R markdown source). We will provide the solutions to instructors upon request.

Python/pandas/scikit-learn handout

The assignment was designed in R/tidyverse, and is a particularly good fit for courses that use R. We provide a Python/pandas/scikit-learn translation. A handout with starter code in Python is here (Jupyter Notebook source). Solutions in Python will be available to instructors upon request.

Lessons learned

Explain the dataset. Some students are excited to work with physiological data, but some are just confused. It may make sense to explain the dataset in class.

Linear regression is difficult. In our variant of the assignment, we asked for students to formulate and test a hypothesis about the data. Students found it quite challenging to find a situation where the assumptions of linear regression were satisfied, though it is possible to do.

Explicitly grade the quality of the report. We strongly recommend that instructors explicitly grade project reports on the quality of the writing. We found that this trick increases the quality of the reports substantially.