Quality Checks

9.2. Quality Checks

Once your data are in a table and you understand the scope and granularity, it’s time to inspect for quality. You may have come across errors in the source as you examined and wrangled the file into a data frame. In this section, we describe how to continue this inspection and carry out a more comprehensive assessment of the quality of the features and their values. We consider quality from four vantage points:

  • Scope: Do the data match your understanding of the population?

  • Measurements and Values: Are the values reasonable?

  • Relationships: Are related features in agreement?

  • Analysis: Which features might be useful in a future analysis?

Quality based on scope

In Chapter 2, we addressed whether or not the data that have been collected can adequately address the problem at hand. There, we identified the target population, access frame, and sample in collecting the data. That framework helps us consider possible limitations that might impact the generalizability of our findings.

While these broader data scope considerations are important as we deliberate our final conclusions, they are also useful for checking data quality. For example, for the San Francisco restaurant inspections data introduced in Chapter 8, a side investigation tells us that zip codes in the city should start with 941. But, a quick check shows that several zip codes begin with other digits:

94621        1
94120        1
94066        1
941033148    1
941102019    1
Ca           1
Name: postal_code, Length: 10, dtype: int64

This cross-check with scope helps us spot potential problems.

As another example, a bit of background reading on atmospheric CO2 reveals that typical measurements are about 400 ppm worldwide. So we can check whether the monthly averages of CO2 at Mauna Loa lie between 300 to 450 ppm.

Quality of measurements and recorded values

We can use also check the quality of measurements by considering what might be a reasonable value for a feature. For example, imagine what might be a reasonable range for the number of violations in a restaurant inspection. Possibly, 0 to 5. Other checks can be based on common knowledge of ranges: a restaurant inspection score must be between 0 and 100; months run between 1 and 12. We can use documentation to tells us the expected values for a feature. For example, the type of emergency room visit in the DAWN survey, introduced in Chapter 8, has been coded as 1, 2, …, 8 (see Figure 9.1). So, we can confirm that all values for the type of visit are indeed integers between 1 and 8.


Fig. 9.1 Screenshot of the description of the CASETYPE variable in the DAWN survey. Notice that there are eight possible values for this feature. And to help in figuring out if we have properly read the data, we can check the counts for these eight values.

We also want to ensure that the data type matches our expectations. For example, we expect a price to be a number, whether or not it’s stored as integer, floating point, or string. Confirming that the units of measurement match what is expected can be another useful quality check to perform (for example weight values recorded in pounds, not kilograms). We can devise checks for all of these situations.

Other checks can be devised by comparing two related features.

Quality across related features

At times two features have builtin conditions on their values that we can cross-check against other features. For example, according to the documentation for the DAWN study, alcohol consumption is only considered a valid reason for a visit to the ER for patients under 21 so we can check that any record that records alcohol for the type of visit has an age under 21. A cross-tabulation of the features type and age can confirm this constraint is met.

display_df(pd.crosstab(dawn['age'], dawn['type']), rows=12)
type 1 2 3 4 5 6 7 8
-8 2 2 0 21 5 1 1 36
1 0 6 20 6231 313 4 2101 69
2 8 2 15 1774 119 4 119 61
3 914 121 2433 2595 1183 48 76 4563
4 817 796 4953 3111 1021 95 44 6188
5 983 1650 0 4404 1399 170 48 9614
6 1068 1965 0 5697 1697 140 62 11408
7 957 1748 0 5262 1527 100 60 10296
8 1847 3411 0 10221 2845 113 115 18366
9 1616 3770 0 12404 3407 75 150 18381
10 616 1207 0 12291 2412 31 169 7109
11 205 163 0 24085 2218 12 308 1537

The cross tabulation confirms that all of the alcohol cases (type is 3) have an age under 21 (these are coded as 1, 2, 3, and 4). The data values are as expected.

One last type of quality check pertains to the amount of information found in a feature.

Quality for analysis

Even when data pass the previous quality checks, problems can arise with its usefulness. For example, if all but a handful of values for a feature are identical, then that feature adds little to the understanding of underlying patterns and relationships. Or, if there are too many missing values, especially if there is a discernible pattern in the missing values, our findings may be limited. And, if a feature has many bad/corrupted values, then we might question the accuracy of even those values that fall in the appropriate range.

We see below that the type of restaurant inspection in San Francisco can be either routine or from a complaint. Since only one of the 14,000+ inspections was from a complaint, we lose little if we drop this feature, and we might also want to drop that single inspection as it represents an anomaly.

routine      14221
complaint        1
Name: type, dtype: int64

Once we find problems with our data, we need to figure out what to do.

Essentially you have four options: leave the data as is; modify values; remove features; or drop records. Not every unusual aspect of the data needs to be fixed. You might have discovered a characteristic of your data that will inform you about how to do your analysis and otherwise does not need correcting. Or, you might find that the problem is relatively minor and most likely will not impact your analysis so you can leave the data as is.

On the other hand, you might want to replace corrupted values with NaN, or you might have figured out what went wrong and correct the value. Other possibilities for modifying records are covered in the examples of Chapter 10. If you plan to change the values of a variable, then it’s good practice to create a new feature with the modified value and preserve the original feature, or at a minimum, create a new feature that indicates which values in the original feature have been modified. These approaches give you some flexibility in checking the influence of the modified values on your analysis.

If you find yourself modifying many values in a feature, then you might consider eliminating that feature entirely. Either way, you will want to study the possible impact of excluding the feature from your analysis. In particular, you will want to determine whether the records with corrupted values are similar to each other, and different from the rest of the data. This would indicate that you may be unable to capture the impact of a potentially useful feature in your analysis. Rather than exclude the feature entirely, there may be a transformation that allows you to keep the feature while reducing the level of detail recorded.

At times, you may want to eliminate the problematic records. In general, we do not want to drop a large number of observations from a dataset without good reason. You may want to scale back your investigation to a particular subgroup of the data, but that’s a different situation than dropping records because of a corrupted value in a field. When you discover that an unusual value is in fact correct, you still might decide to exclude the record from your analysis because it’s so different from the rest of your data and you do not want it to overly influence your analysis.

Quality checks can reveal issues in the data that need to be addressed before proceeding with analysis. One particularly important type of check is to look for missing values. We suggested that there may be times when you want to replace corrupted data values with NaN, and hence treat them as missing. At other times, data might arrive missing. What to do with missing data is an important topic and there is a lot of research on this problem; we cover ways to address missing data in the next section.