5 minute read

In this blog post I want to show you how to calculate approximate confidence intervals in SQL (Presto/Athena dialect) using bootstrapping, specifically for the Root Mean Squared Error (RMSE) and in multiple subgroups.

This post builds on top of this blog post, Bootstrap Confidence Intervals in SQL for PostgreSQL and BigQuery, but extends it for multiple subgroups.

Final result

Starting from some data with columns (category, label, error) where err is the prediction error calculated as correct minus your predicted probability (err = correct - prediction)

category label err
0 0 1
0 0 2
0 0 3
0 1 11
0 1 12
0 1 13
0 1 14
1 0 100
1 0 200
1 0 300
1 0 400
1 0 500

The query calculates the bootstrapped 95% RMSE CI

category label rmse_lo rmse rmse_hi
0 0 1.0 2.16 3.0
0 1 11.53 12.55 13.28
1 0 223.61 331.66 397.49

How

Let’s jump right in!

with labelled as (
	select *
	from (
        values (0, 0,1),
            (0, 0, 2),
            (0, 0, 3),
            (0, 1, 11),
            (0, 1, 12),
            (0, 1, 13),
            (0, 1, 14),
            (1, 0, 100),
            (1, 0, 200),
            (1, 0, 300),
            (1, 0, 400),
            (1, 0, 500)
		) as t(category, label, err)
),
bootstrap_indexes as (
SELECT bootstrap_index
	FROM (SELECT sequence(1, 10) as d)
	CROSS JOIN UNNEST (d) as t(bootstrap_index)
),
bootstrap_data AS (
  SELECT 
        category,
        label,
        err,
        ROW_NUMBER() OVER(partition by category, label) - 1 AS data_index
  FROM labelled
),
bootstrap_amounts as (
    select
        category,
        label,
        count(*) as amount
    from bootstrap_data
    group by category, label
),
bootstrap_map AS (
  SELECT 
    d.category,
    d.label,
    a.amount,
    cast(floor(random() * a.amount) as int) AS sampled_index,
    bootstrap_index
  FROM bootstrap_data d
  JOIN bootstrap_amounts a 
    on a.category = d.category 
    and a.label = d.label
  JOIN bootstrap_indexes ON TRUE
), 
bootstrap AS (
  SELECT 
    m.category,
    m.label,
    bootstrap_index,
    err
  FROM bootstrap_map m
  JOIN bootstrap_data d 
    on m.category = d.category 
    and m.label = d.label 
    and m.sampled_index = d.data_index
),
bootstrap_aggregated AS (
  SELECT 
    category,
    label,
    bootstrap_index,
    sqrt(avg(pow(err, 2))) as rmse
  FROM bootstrap
  group by category, label, bootstrap_index
),
bootstrap_ci AS (
  SELECT
    category,
    label,
    approx_percentile(rmse, 0.025) as rmse_lo,
    approx_percentile(rmse, 0.975) as rmse_hi
  FROM bootstrap_aggregated
  group by category, label
),
sample AS (
  SELECT 
    category,
    label,
    sqrt(avg(pow(err, 2))) AS rmse_avg
  FROM labelled
  group by category, label
)
SELECT 
    s.category,
    s.label,
    round(rmse_lo,2) as rmse_lo,
    round(rmse_avg, 2) as rmse,
    round(rmse_hi, 2) as rmse_hi
FROM sample s
JOIN bootstrap_ci c 
    on s.category = c.category 
    and s.label = c.label
order by category, label

Let’s break this query down one CTE at a time.

labelled creates our data.

category label err
0 0 1
0 0 2
0 0 3
.. ..
1 0 500

bootstrap_indexes enumerates the bootstrap samples you want to do.

bootstrap_index
1
9
10

bootstrap_data enumerates each row in each category/label subgroup. We need this query because we are going to use data_index to resample.

category label err data_index
0 1 11 0
0 1 12 1
0 1 13 2
0 1 14 3
0 0 1 0
0 0 2 1
0 0 3 2
1 0 100 0
1 0 200 1
1 0 300 2
1 0 400 3
1 0 500 4

bootstrap_amounts contains the number of rows in each category/label subgroup.

category label amount
0 1 4
0 0 3
1 0 5

bootstrap_map performs the resampling with replacement.

For each category/label subgroup we take a full sample by sampling random integers in the range of amount (which differs per subgroup).

The JOIN bootstrap_indexes ON TRUE gives the full cartesian product of the bootstrap and data indexes, in this case we have 10 bootstrap samples and 12 observations which results in 120 rows.

category label amount sampled_index bootstrap_index
0 0 3 0 1
0 0 3 2 1
0 0 3 1 1
0 1 4 3 1
.. .. .. .. ..
0 1 3 1 9

bootstrap joins back the actual data instead of the sampled index. This bootstrap table now contains 10 full new samples we bootstrapped!

category label bootstrap_index err
0 0 1 1
0 0 1 1
0 0 1 1
0 1 1 12
.. .. .. ..
0 1 9 13

bootstrap_aggregated calculates the RMSE for each individual sample (i.e. bootstrap_index).

category label bootstrap_index rmse
0 0 2 3.0
0 0 4 2.71
0 0 5 2.71
0 0 9 1.41
0 0 1 2.16
0 0 6 2.52
0 0 8 2.16
0 0 10 2.16
1 0 4 428.95
0 1 2 13.04
.. .. .. ..
1 0 10 240.83

bootstrap_ci uses approx_percentile() to find the percentiles of the empirical bootstrap distribution that we need.

category label rmse_lo rmse_hi
0 0 1.41 2.52
1 0 249.0 500.0
0 1 11.51 13.53

sample computes the actual observed RMSE in the category/label subgroups. Note that I calculate the RMSE from the actual observations here and not the bootstrapped RMSE!

category label rmse_avg
0 1 12.55
0 0 2.16
1 0 331.66

Finally, we can combine everything together: the bootstrapped RMSE 95% CI and the observed RMSE for every category/label subgroup.

category label rmse_lo rmse rmse_hi
0 0 1.41 2.16 2.71
0 1 11.51 12.55 13.51
1 0 249.0 331.66 428.95

Optional: Simple case (without subgroups)

For the simple case (without subgroups) I want to refer you back to the original blog post.

For completeness I’ll add it in. Imagine you have a table with prediction errors like this:

err
100
200
300
400
500

And you want to turn it into this (where rmse_lo and rmse_hi are bootstrapped RMSE 95% CI.):

rmse_lo rmse rmse_hi
279.28 331.66 426.61
with labelled as (
	select *
	from (
			values 
				(100),
				(200),
				(300),
				(400),
				(500)
		) as t(err)
),
bootstrap_indexes as (
SELECT bootstrap_index
	FROM (SELECT sequence(1, 10) as d)
	CROSS JOIN UNNEST (d) as t(bootstrap_index)
),
bootstrap_data AS (
  SELECT 
        err,
        ROW_NUMBER() OVER() - 1 AS data_index
  FROM labelled
),
bootstrap_map AS (
  SELECT 
    cast(floor(random() * (select count(data_index) from bootstrap_data)) as int) AS sampled_index,
    bootstrap_index
  FROM bootstrap_data d
  JOIN bootstrap_indexes ON TRUE
), 
bootstrap AS (
  SELECT 
    bootstrap_index,
    err
  FROM bootstrap_map m
  JOIN bootstrap_data d on m.sampled_index = d.data_index
),
bootstrap_aggregated AS (
  SELECT 
    bootstrap_index,
    sqrt(avg(pow(err, 2))) as rmse
  FROM bootstrap 
  group by bootstrap_index
),
bootstrap_ci AS (
  SELECT
    approx_percentile(rmse, 0.025) as rmse_lo,
    approx_percentile(rmse, 0.975) as rmse_hi
  FROM bootstrap_aggregated
),
sample AS (
  SELECT 
    sqrt(avg(pow(err, 2))) AS rmse_avg
  FROM labelled
)
SELECT 
    round(rmse_lo,2) as rmse_lo,
    round(rmse_avg, 2) as rmse,
    round(rmse_hi, 2) as rmse_hi
FROM sample s
JOIN bootstrap_ci on TRUE

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