Background

This document is an abbreviated attempt at addressing some of the issues related to accuracy and uncertainty that I have brought up over discussion of raster (soil class) mapping standards. As such, the following is a combination of soap box moments, demonstrations of methods, todo items for my future self, and references. Honestly, before going any further be sure to read the recent paper by Rossiter, Zeng, and Zhang (2017).

This article does a fine job of summarizing the major differences between classification and prediction. Most of the modeling frameworks we will be using or evaluating generate predictions in the form of probabilities (prediction). At some point the stack of probabilities will be converted into a single map depicting the most likely class at each pixel (classification). The iterative assessment of model performance (e.g. accuracy and uncertainty) should happen as part of the prediction phase via metrics such as the Brier score and Shannon entropy. An evaluation of the final classification is helpful for communicating accuracy to a wider audience (e.g. percent correctly classified) but should not be the primary means of optimizing model performance.

Status Quo

The "Digital Soil Mapping" chapter (5) from the latest Soil Survey Manual describes two commonly used metrics for the description of accuracy and uncertainty: overall accuracy / percent correctly classified (PCC) and the confusion index (CI) of Burrough, van Gaans, and Hootsmans (1997). These methods are widely used and implementation is simple.

Given the complex nature of class mapping results (e.g. stack of class probabilities) and inherent (likely quantifiable) similarity of soil classes, I think that we should explore options for a more robust suite of accuracy and uncertainty metrics. Furthermore, it is my opinion that any evaluation of probabilistic predictions should be integrated over all classes.

Theses

  • The \(\tau\) statistic of (Rossiter, Zeng, and Zhang 2017) is a more reliable and nuanced representation of accuracy vs. PCC.

  • The \(\tau\) statistic can be upgraded with additional knowledge given the availability of 1) prior understanding of class proportions, and/or, 2) meaningful parameterization of pair-wise class distances.

  • There can be consensus on formulation of approximate pair-wise distances, within a given modeling domain. Pair-wise distances may not necessarily be the same across modeling domains or projects.

  • Brier scores are option for an even more nuanced representation of accuracy as they integrate all predicted probabilities.

  • The confusion index of Burrough, van Gaans, and Hootsmans (1997) is an unstable metric when the number of predicted classes is large and when the most likely classes are associated with low probabilities.

  • Shannon entropy (log base 2) is a more reliable representation of uncertainty than the confusion index, especially when the number of possible classes varies by project. The importance of a universally reliable representation of uncertainty is even more important when several methods are used concurrently.

  • There should be a way to integrate pair-wise distances into the Shannon entropy (or related method) and Brier scores; maybe we will discover those here.

Soap Box Time

Our current QC/QA process is based on many forms of evaluation, accumulates some degree of subjectivity and relies heavily on qualitative forms of information (field experience, institutional knowledge, etc.). On the opposite side of the spectrum, the validation of raster mapping is often claimed to be free of subjective interference and entirely quantitative. Those are "good things" that we should always strive for, however, the simplicity of calculating a "percent correctly classified" can interfere with a more nuanced evaluation of accuracy. As I mentioned on the phone (and implicitly volunteered for) a validation "score" might be more meaningful than any single validation metrics.

One such score might include:

  • agreement between predicted probabilities and observed class (e.g. Brier scores)
  • agreement between the most likely class and observed class, accounting for class similarities (e.g. weighted \(\tau\))
  • distribution of class-wise Shannon entropy values
  • calibration vs. predicted vs. validation proportion of classes
  • some kind of metric that integrates spatial connectivity of predictions / observations, for example: cross-tabulate calibration / prediction / validation classes with geomorphon classes

I strongly believe that we need a robust suite of metrics primarily for internal discussion and evaluation of raster mapping products; even more so when complex modeling frameworks such as randomForest or neural nets are used.

Accuracy and uncertainty metrics are primarily vehicles for understanding, re-calibrating (as needed), and communicating statistical models as part of the development and QA/QC process.

Concept Demonstration via Simulated Data

Consider a supervised classification that generates predictions for 5 possible soil classes. Suites of predicted probabilities fall into 3 general cases:

Probability distributions of predictions.

Probability distributions of predictions.

Even though these are simulated data, the three cases above demonstrate common modeling scenarios where classification uncertainty ranges from very low ("Case 3") in some areas to quite high ("Case 1") in others. These three cases could easily be associated with real situations:

Accuracy

Confusion Matrix / Area Under ROC

See Chapter 9.

Review some of the commentary on the use of only the confusion matrix and AUROC for rating predictions in the medical field.

The confusion matrix and associated statistics are a useful starting point but not the most effective means for comparing performance.

Brier Scores

Brier scores (Brier 1950, Harrell (2001)) quantify agreement between observed classes and predicted probabilities: \[ B = \frac{1}{n} \sum_{i=1}^{n}{ ( p_{i} - y_{i} )^{2} } \] where \(B\) is an index of agreement between predicted probabilities, \(\mathbf{p}\), and class labels, \(\mathbf{y}\). Larger values suggest less agreement between probabilities and observed class labels.

Follow-up:

What about a weighted version of this score, based on a re-statement of the distance matrix?

Tau and Weighted Tau (class-similarity)

(Rossiter, Zeng, and Zhang 2017) implemented in aqp::tauw(). This paper contains some discussion on a weighted version of Shannon Entropy using the subset of similarities between predicted classes and the actual class.

Commentary from DGR

  • Prior class probabilities. Commentary from DGR:
    • That depends on the mapping method. In LDA we can set the priors, then we'd use these in tau. But for an automatic DSM procedure the priors are all equal (Foody's modified kappa). If judging a manual mapper, the priors can be their overall probabilities for an area. E.g., in one county we have a pretty good idea that it is half Vertisols, so the mapper is prejudiced (in the good sense) about this.
  • Class similarity
    • The weighting is quite tricky since obviously it can be used to manipulate results. I really like the 'error loss' method if there is some numerical value put on each difference -- as I did with the NC site index. In CA you have the Storie index, you could use that difference for mis-mappings of series. Numerical taxonomy measures could also be used but you'd need to agree on which properties to use. If the purpose of the map is e.g. to estimate C stocks, then the difference between the mean C stocks between classes from NASIS might be used. Coming up with a transparent and accepted weighting can be tricky.

Uncertainty

Shanon Entropy

\[ H = -\sum_{i=1}^{n}{p_{i} * log_{2}(p_{i})} \]

where \(H\) is an index of uncertainty associated with predicted probabilities, \(\mathbf{p}\), of encountering classes \(i\) through \(n\). Smaller values imply less entropy (more information). Given equal class probabilities, H will increas as the number of classes increases.

Kempen et al. (2009) described a normalized version of Shannon entropy that is constrained to values between 0 and 1:

\[ H = -\sum_{i=1}^{n}{p_{i} * log_{n}(p_{i})} \] where \(H\) is an index of uncertainty associated with predicted probabilities, \(\mathbf{p}\), of encountering classes \(i\) through \(n\). This representation may be conveniently contained within the range of \([0,1]\), however, it cannot be used to compare uncertainty from models using different numbers of classes.

It is my recommendation that the \(log_{2}\) version of Shannon H be used as our primary metric of uncertainty for predictive soil class mapping.

Shannon entropy does not take into account similarity among classes.

Review

example brier.score tau.equal tau.actual PCC
Case 1 0.73 0.20 0.14 0.36
Case 2 0.73 0.25 0.20 0.40
Case 3 0.32 0.77 0.43 0.82
Class Probabilities
Uncertainty
example A B C D E Shannon.H
Case 1 0.06 0.23 0.13 0.25 0.33 2.14
Case 2 0.13 0.11 0.23 0.10 0.44 2.07
Case 3 0.03 0.05 0.02 0.08 0.83 0.97

Example Implementation

The aqp package has an implementation of Shannon entropy and Brier score; there are many other implementations but these are convenient for soil survey work. Consider the following table of predicted probabilities (classes A,B,C,D,E) and observed class (actual).

library(aqp)

# example data
d <- structure(list(A = c(0.0897243494322252, 0.0537087411977284, 
0.0643087579284512, 0.0582791533521884, 0.0655491726966812, 0.0878056947034425, 
0.0550727743006022, 0.10724015754623, 0.0332599961787985, 0.0555131608754956
), B = c(0.191110141078936, 0.187244044389649, 0.119214057525671, 
0.198461646003737, 0.161851348940294, 0.172157251906694, 0.113611770097243, 
0.178697159594029, 0.194607795787689, 0.188977055949146), C = c(0.121941735763077, 
0.0770539012535731, 0.0977753159795662, 0.0774293724263895, 0.072198187957068, 
0.0366921003115242, 0.151033286139089, 0.0974443429098862, 0.124876574685048, 
0.0864142563046045), D = c(0.351108807309283, 0.322120077305279, 
0.440632731639948, 0.401063395801608, 0.312647702445919, 0.304193047630158, 
0.270239142407351, 0.258895264130713, 0.422747316475851, 0.252724366285052
), E = c(0.246114966416479, 0.359873235853771, 0.278069136926363, 
0.264766432416077, 0.387753587960038, 0.399151905448182, 0.410043027055715, 
0.357723075819142, 0.224508316872614, 0.416371160585702), id = c("1", 
"10", "100", "1000", "101", "102", "103", "104", "105", "106"
), actual = c("D", "B", "D", "E", "D", "D", "E", "E", "D", "E"
)), .Names = c("A", "B", "C", "D", "E", "id", "actual"), row.names = c(NA, 
10L), class = "data.frame")

# check it out
# predictions, and actual, observed class
head(d)
##            A         B          C         D         E   id actual
## 1 0.08972435 0.1911101 0.12194174 0.3511088 0.2461150    1      D
## 2 0.05370874 0.1872440 0.07705390 0.3221201 0.3598732   10      B
## 3 0.06430876 0.1192141 0.09777532 0.4406327 0.2780691  100      D
## 4 0.05827915 0.1984616 0.07742937 0.4010634 0.2647664 1000      E
## 5 0.06554917 0.1618513 0.07219819 0.3126477 0.3877536  101      D
## 6 0.08780569 0.1721573 0.03669210 0.3041930 0.3991519  102      D

Brier scores (accuracy) are computed over all predictions and associated observed classes.

# compute Brier score from all predictions
brierScore(d, classLabels = c('A', 'B', 'C', 'D', 'E'), actual = 'actual')
## [1] 0.5833992

Shannon entropy (uncertainty) is computed from each vector of predicted probabilities.

# shannon entropy for first row, could be a single pixel or obs. point
shannonEntropy(d[1, c('A', 'B', 'C', 'D', 'E')])
## [1] 2.166525
# compute shannon entropy for all rows
apply(d[, c('A', 'B', 'C', 'D', 'E')], 1, shannonEntropy)
##        1        2        3        4        5        6        7        8        9       10 
## 2.166525 2.021157 1.982791 2.024063 2.011094 1.971243 2.036219 2.151995 2.006615 2.018874

Resources

References

Brier, GLenn W. 1950. “Verification of Forecasts Expressed in Terms of Probability.” Monthly Weather Review 78 (1): 1–3. doi:10.1175/1520-0493(1950)078<0001:VOFEIT>2.0.CO;2.

Burrough, P.A., P.F.M. van Gaans, and R. Hootsmans. 1997. “Continuous Classification in Soil Survey: Spatial Correlation, Confusion and Boundaries.” Geoderma 77: 115–35. doi:10.1016/S0016-7061(97)00018-9.

Harrell, Frank E. 2001. Regression Modeling Strategies. Springer Series in Statistics. New York, NY: Springer.

Kempen, Bas, Dick J. Brus, Gerard B.M. Heuvelink, and Jetse J. Stoorvogel. 2009. “Updating the 1:50,000 Dutch Soil Map Using Legacy Soil Data: A Multinominal Logistic Regression Approach.” Geoderma 151: 311–26. doi:10.1016/j.geoderma.2009.04.023.

Rossiter, David G., Rong Zeng, and Gan-Lin Zhang. 2017. “Accounting for Taxonomic Distance in Accuracy Assessment of Soil Class Predictions.” Geoderma 292 (Supplement C): 118–27. doi:10.1016/j.geoderma.2017.01.012.