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Difference between blocking_rules_to_generate_predictions vs blocking rules for estimation

What is the difference between the list of blocking_rules_to_generate_predictions specifed in the Splink settings dictionary, and the blocking rule that must be provided as an argument to estimate_parameters_using_expectation_maximisation?

These two kinds of blocking rules can be seen in the following code snippet:

settings = {
    "link_type": "dedupe_only",
    "blocking_rules_to_generate_predictions": [
        "l.first_name = r.first_name and substr(l.surname,1,1) = substr(r.surname,1,1)",
        "l.dob = r.dob",
    ],
    "comparisons": [
        levenshtein_at_thresholds("first_name", 2),
        exact_match("surname"),
        exact_match("dob"),
        exact_match("city", term_frequency_adjustments=True),
        exact_match("email"),
    ],
}


linker = DuckDBLinker(df, settings)
linker.estimate_u_using_random_sampling(target_rows=1e6)

blocking_rule_for_training = "l.first_name = r.first_name and l.surname = r.surname"
linker.estimate_parameters_using_expectation_maximisation(blocking_rule_for_training)

blocking_rule_for_training = "l.dob = r.dob and l.city = r.city"
linker.estimate_parameters_using_expectation_maximisation(blocking_rule_for_training)

The answer is that they serve different purposes.

What is a blocking rule?

Blocking rules are needed because it is usually computationally intractable to compare every record with every other.

A blocking rule specifies a constraint on how Splink generates pairwise record comparisons, dramatically reducing the total number of comparisons generated.

For example, the blocking rule "l.first_name = r.first_name and l.surname = r.surname" will generate pairwise record comparisons amongst pairwise comparisons where first name and surname match.

The purpose of blocking_rules_to_generate_predictions

blocking_rules_to_generate_predictions are used by Splink when the user called linker.predict().

The purpose of these blocking rules is to try and ensure that pairwise record comparisons are generated for all true matches.

For example,

settings = {
    "blocking_rules_to_generate_predictions" [
        "l.first_name = r.first_name and l.surname = r.surname"
        ]
}

will generate comparisons for all true matches where names match. But it would miss a true match where there was a typo in (say) the first name.

In general, it is usually impossible to find a single rule which both:

  • Reduces the number of comparisons generated to a computatally tractable number

  • Ensures comparisons are generated for all true matches

This is why blocking_rules_to_generate_predictions is a list. Suppose we also block on postcode:

settings = {
    "blocking_rules_to_generate_predictions" [
        "l.first_name = r.first_name and l.surname = r.surname",
        "l.postcode = r.postcode"
        ]
}

We will now generate a pairwise comparison for the record where there was a typo in the first name, so long as there isn't also a difference in the postcode.

By specifying a variety of blocking_rules_to_generate_predictions, it becomes implausible that a truly matching record would not be captured by at least one of the rules.

Note that Splink automatically deduplicates the record comparisons it generates. So, in the example above, the "l.postcode = r.postcode" blocking rule generates only records comparisons that were not already captured by the first_name and surname rule.

The purpose of the blocking_rule parameter on estimate_parameters_using_expectation_maximisation

The purpose of this blocking rule is to reduce the number of pairwise generated to a computationally-tractable number to enable the expectation maximisation algorithm to work.

The expectation maximisation algorithm seems to work best when the pairwise record comparisons are a mix of anywhere between around 0.1% and 99.9% true matches. It works less effectively if there are very few examples of either matches or non-matches. It works less efficiently if there is a huge imbalance between the two (e.g. a billion non matches and only a hundred matches).

It does not matter if this blocking rule excludes some true matches - it just needs to generate examples of matches and non matches.

Since they serve different purposes, the blocking rules most appropriate to use with blocking_rules_to_generate_predictions will often be different to those for estimate_parameters_using_expectation_maximisation, but it is also common for the same rule to be used in both places.