The CategoricalCAP measures the risk of disclosing sensitive information through an inference attack. We assume that some values in the real data are public knowledge. An attacker is combining this with synthetic data to make guesses about other real values that are sensitive.
This metric describes how difficult it is for an attacker to correctly guess the sensitive information using an algorithm called Correct Attribution Probability (CAP).
- Categorical: This metric is meant for discrete, categorical data
- Boolean: This metric works well on boolean data
(best) 1.0: The real data is 100% safe from the attack. The attacker is not able to correctly guess any of the sensitive values by applying the CAP algorithm.
(worst) 0.0: The real data is not at all safe from the attack. The attacker is able to correctly guess every sensitive value by applying the CAP algorithm.
Scores between 0.0 and 1.0 indicate the overall safety of the real data. For example a score of 0.55 indicates that the data is generally 55% safe from attack.
This image shows the some sensitive real data in comparison to an attacker's guesses. The probability of the attacker correctly guessing the real value is highlighted. We can use this to determine the overall safety of the data. In this case, the CategoricalCAP score is 0.55, meaning the data is on average 55% safe from attack.
We assume that the attacker is in possession of
- few columns of the real data (
key_fields), as well as
- the full synthetic dataset, including synthetic sensitive values
The attacker's goal is to correctly guess the real value of the sensitive information,
sensitive_fields. An example is shown below.
In this example, we assume the
key_fieldsare a person's age bracket and gender. Meanwhile, the
sensitive_fieldsare the person's political affiliation; this is what the attacker wants to guess.
In this metric, the attacker uses an algorithm called CAP , summarized below.
The attacker follows 4 steps to guess a sensitive value.
- 1.Pick a row (r) in the real dataset. Note down all the
- 2.In the synthetic data, find all the rows that match the
key_fieldsof r. Call this set of rows, S. The set S is also known as the (synthetic) equivalence class of r.
- 3.Each row in S will have synthetic values for the
sensitive_fields. Let each of the values vote to guess the
sensitive_fieldsof the real row, r.
- 4.The final score is the frequency of votes that are actually correct for all sensitive fields. This value that must be between 0 and 1.
An illustration of this algorithm is shown below.
If a real row has an age bracket of
20-29and a gender of
F, the synthetic equivalent class (S) for it has the same. In this case, S has 4 rows. Each has a synthetic value for the sensitive
We repeat the attack for all rows (r) in the real data and calculate an overall probability of guessing the sensitive column correctly. The metric returns
1-probabilityso that a higher score means higher privacy.
The SDMetrics library supports variants CAP algorithm, based on what the attacker can do with the equivalence class.
- CategoricalZeroCAP: If there are no rows in the equivalence class, then the attacker records a failure (
0) to guess that row
- CategoricalGeneralizedCAP: If there are no rows in the equivalence class, the attacker looks for closest matches instead of exact matches . Because the
key_fieldsare all discrete, the attacker finds the closest matches by using the hamming distance . This ensures that there is always at least 1 row in the equivalence class, which means that there is always at least 1 guess.
Access this metric from the
single_tablemodule and use the
from sdmetrics.single_table import CategoricalCAP
score = CategoricalCAP.compute(
real_data: A pandas.DataFrame containing the real data
synthetic_data: A pandas.DataFrame containing the same columns of synthetic data
key_fields: A list of strings representing the column names that the attacker already knows. These must be compatible with the metric.
sensitive_fields: A list of string representing the column names that the attacker wants to guess. These must be compatible with the metric.
from sdmetrics.single_table import CategoricalZeroCAP, CategoricalGeneralizedCAP
score = CategoricalZeroCAP.compute(