An online system obtains a set of resolved impressions based on historical data about multiple publishers. A set of features is then extracted, for each resolved impression, based on a comparison of historical data about the first publisher and the second publisher. The online system performs training of a machine-learned model based on the set of features. Data about a plurality of new impressions are input into the trained machine-learned model to obtain an output of the trained machine-learned model. A reach overlap metric and unique reach metric can be computed based on the output of the trained machine-learned model.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method comprising: receiving data about a training set of impressions that were provided via a first publisher and were provided to users of an online system who did not have any impressions of content items via a second publisher; obtaining, for each impression in the training set of impressions, a set of features as a function of a comparison of data about the first publisher and the second publisher; training a machine-learned model for estimation of a number of users reached for presentation of content via a plurality of impressions, based on the set of features obtained for each impression in the training set of impressions; inputting data about the plurality of impressions into the trained machine-learned model to obtain an output of the trained machine-learned model; and computing a reach overlap metric based on the output of the trained machine-learned model. 2. The method of claim 1, wherein the data about the first publisher and the second publisher used for obtaining the set of features are selected from the group consisting of: aggregated statistics on impressions related to the first publisher and the second publisher, statistics of different users of the first publisher and the second publisher, information about different cookies between the first publisher and the second publisher, a percentage of distinct Internet Protocol (IP) addresses of users reached by the first publisher and by the second publisher, and a percentage of same IP addresses of users reached by both the first and the second publishers. 3. The method of claim 1, wherein training the machine-learned model for estimation of the number of users reached for presentation of the content comprises: training the machine-learned model for estimation of a percentage of users reached only by the first publisher for presentation of the content. 4. The method of claim 1, wherein computing the reach overlap metric based on the output of the trained machine-learned model comprises: computing a percentage of users reached only by the first publisher for presentation of the content. 5. The method of claim 4, further comprising: multiplying the computed percentage of users reached only by the first publisher with an estimated total number of users reached by the first publisher to compute a number of users reached only by the first publisher for presentation of the content. 6. The method of claim 4, further comprising: computing an estimated number of common users reached by the first publisher and the second publisher for presentation of the content, based on the computed percentage of users reached only by the first publisher and an estimated total number of users reached by the first publisher. 7. The method of claim 1, wherein computing the reach overlap metric based on the output of the trained machine-learned model comprises: computing a number of users reached only by the first publisher for presentation of the content. 8. The method of claim 1, wherein training the machine-learned model for estimation of the number of users reached for presentation of the content comprises: training the machine-learned model based on at least one of the linear regression algorithm, or one or more other regression techniques. 9. The method of claim 1, wherein training the machine-learned model for estimation of the number of users reached for presentation of the content comprises: training the machine-learned model based on a metric obtained using the training set of impressions. 10. The method of claim 1, further comprising: performing de-synchronization of the training set of impressions; and training the machine-learned model for estimation of the number of users reached for presentation of the content, based on the desynchronized set of impressions. 11. The method of claim 1, comprising: estimating, based on the trained machine-learned model, a first number of users reached by the first publisher for presentation of the content; estimating, based on the trained machine-learned model, a second number of users reached by the second publisher for presentation of the content; estimating, based on the trained machine-learned model, a third number of users reached by a publisher that comprises the first publisher and the second publisher for presentation of the content; computing an estimated number of common users reached by the first publisher and the second publisher for presentation of the content, based on the estimated first number of users, the estimated second number of users and the estimated third number of users. 12. A method comprising: receiving a machine-learned model for estimation of a number of users reached for presentation of content via a plurality of impressions, the machine-learned model being trained based on a set of features obtained for each impression in a training set of impressions as a function of a comparison of data about a first publisher and a second publisher, the training set of impressions were provided via the first publisher and were provided to users of an online system who did not have any impressions of content items via the second publisher; inputting data about the plurality of impressions into the trained machine-learned model to obtain an output of the trained machine-learned model; and computing a reach overlap metric based on the output of the trained machine-learned model. 13. The method of claim 12, wherein computing the reach overlap metric based on the output of the trained machine-learned model comprises: computing a percentage of users reached only by the first publisher for presentation of the content. 14. A computer program product comprising a computer-readable storage medium having instructions encoded thereon that, when executed by a processor, cause the processor to: receive data about a training set of impressions that were provided via a first publisher and were provided to users of an online system who did not have any impressions of content items via a second publisher; obtain, for each impression in the training set of impressions, a set of features as a function of a comparison of data about the first publisher and data about the second publisher; train a machine-learned model for estimation of a number of users reached for presentation of content via a plurality of impressions, based on the set of features obtained for each impression in the training set of impressions; input data about the plurality of impressions into the trained machine-learned model to obtain an output of the trained machine-learned model; and compute a reach overlap metric based on the output of the trained machine-learned model. 15. The computer program product of claim 14, wherein train the machine-learned model for estimation of the number of users reached for presentation of the content comprises: train the machine-learned model for estimation of a percentage of users reached only by the first publisher for presentation of the content. 16. The computer program product of claim 14, wherein compute the reach overlap metric based on the output of the trained machine-learned model comprises: compute a percentage of users reached only by the first publisher for presentation of the content. 17. The computer program product of claim 16, wherein the instructions further cause the processor to: multiply the computed percentage of users reached only by the first publisher with an estimated total number of users reached by the first publisher to compute a number of users reached only by the first publisher for presentation of the content. 18. The computer program product of claim 16, wherein the instructions further cause the processor to: compute an estimated number of common users reached by the first publisher and the second publisher for presentation of the content, based on the computed percentage of users reached only by the first publisher and an estimated total number of users reached by the first publisher. 19. The computer program product of claim 14, wherein compute the reach overlap metric based on the output of the trained machine-learned model comprises: compute a number of users reached only by the first publisher for presentation of the content. 20. The computer program product of claim 14, wherein train the machine-learned model for estimation of the number of users reached for presentation of the content comprises: train the machine-learned model based on at least one of the linear regression algorithm, or one or more other regression techniques.