ICE Book-ins Closely Match Immigration Arrests from Deportation Data Project (Part 4)

In Part 4 of my series on ICE data from the Deportation Data Project, I find that ICE book-ins correspond closely to ICE administrative arrests—a simple but hopefully useful observation.

This is Part 4 of a multi-part series on ICE data made public by the Deportation Data Project. If you haven't yet, you may want to read Part 1 (or listen), Part 2, or Part 3 before diving into this analysis.

There has been a longstanding discussion in immigration research about whether ICE detention book-ins actually correspond to ICE administrative arrests. While these are technically different data points that should not be conflated, after analyzing both datasets side by side, I find that ICE detention book-ins and administrative arrests correspond remarkably closely.

This is Part 4 of my ongoing analysis of the detailed immigration enforcement data released by the Deportation Data Project, and it represents a shorter, more focused examination of a specific methodological question that has implications for how we interpret and use this data going forward.

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The Conceptual Distinction

From a definitional perspective, an administrative arrest and a detention book-in remain distinct events. An arrest occurs when ICE takes someone into custody for a civil violation of immigration law. A book-in occurs when that person is processed into a detention facility. In theory, there could be various scenarios where these two events don't align one-to-one—someone could be arrested and released without detention, transferred between facilities, or processed through alternative custody arrangements.

It is difficult to imagine conceptually how an ICE officer could arrest someone without that arrest resulting in a book-in into a detention facility, but edge cases certainly exist. This is why people have sometimes referred to book-ins as "ICE arrests" in shorthand, despite the technical distinction.

Comparing the Numbers

I am still working through different parts of the Deportation Data Project dataset, trying to show different observations and findings while demonstrating how to process this data for those who are curious about the methodology. I simply want to show that if we look at total monthly book-ins by ICE into detention facilities and compare that to the aggregate monthly number of administrative arrests over the course of this fiscal year so far, we find that they correspond very closely.

The graph below illustrates this comparison as clearly as possible—not exact correspondence, but very close. Thus, thanks to the Deportation Data Project, we have more confidence than we did before about how to continue to estimate arrests in the absence of new arrest data.

The alignment is close enough that I think it's reasonable to say that, at least at this point in time, book-ins are an appropriate approximation for the number of arrests. This has practical implications for researchers, journalists, and advocates who may have access to one type of data but not the other, or who need to work with historical data where only one metric is available.

Why This Matters

This finding addresses a methodological concern that has persisted throughout immigration enforcement analysis. Detention data and arrest data have often been treated as interchangeable in public discourse, but careful researchers have rightfully questioned whether this conflation introduces errors into our understanding of enforcement patterns.

The close correspondence between these metrics during the current fiscal year suggests that, for this time period at least, the practical difference between arrests and book-ins may be less significant than the conceptual distinction would suggest. This doesn't mean we should abandon precision in our definitions, but it does provide some confidence for analyses that rely on detention book-in data as a proxy for arrest activity.

Using this approach, we can say with more confidence than previously that ICE arrests and CBP arrests (at least those resulting in detention) declined in July compared to June.

Limitations and Future Research

I want people to remain aware that from a definitional perspective, arrests and book-ins are not necessarily the same thing, even if they approximate closely enough to be used interchangeably for current analysis. The relationship between these metrics could change over time, particularly if ICE policies shift toward different custody arrangements or if capacity constraints force changes in detention practices.

I would be interested in conducting a longer historical comparison to see whether this close correspondence holds across different time periods and administrations, but I believe it's better to publish this finding now rather than wait until I have time to conduct a more thorough analysis. Understanding the relationship between these key metrics helps inform how we interpret the broader patterns in immigration enforcement data.

As always, feel free to reach out with questions, comments, or critiques. Methodological transparency and peer review are essential components of responsible data analysis, particularly when dealing with enforcement data that has such significant implications for communities and policy.

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This newsletter is only possible because of your support. If you believe in keeping this work free and open to the public, consider becoming a paid subscriber. You can read more about the mission and focus of this newsletter and learn why, after three years, I finally decided to offer a paid option. If you already support this newsletter financially, thank you.