Rep. Betty Poirier wrote on Thursday that "In 2008, there were 806 illegal immigrants in Massachusetts Correctional Institutions, incarcerated for an average of 241 days. Thus, if the annual cost per inmate in 2008 was $46,000, then the total cost to the taxpayers for keeping these criminals in the United States is over $30 million."

Rep. Poirier's math is wrong; she made a mistake common to many people.

The average cost of an inmate is about $46,000 a year, but that covers both the fixed costs and the variable costs. Massachusetts spends about $9,000 on each inmate's "variable costs," and about $37,000 on "fixed costs." The difference is that variable costs only cover inmates' food, clothing and other things that cost taxpayers money on an inmate-by-inmate basis. Fixed costs, however, such as lighting or correctional officers' salaries, don't change with each additional inmate.

With each additional inmate staying one year, we don't spend $46,000, we spend $9,000.

If we assume that there were 806 illegal immigrants with an average length of stay of 241 days, this amounts to just over $4.8 million, nowhere near the $30 million Rep. Poirier noted. Only if we were able to close a prison or a wing of a prison would we be able to save money on that scale.

Furthermore, I would call into question her number of 806 illegal immigrants. Since she is using a figure of $46,000 per inmate (a Massachusetts Department of Corrections figure) I am assuming that she is excluding county jails and considering just DOC inmates.

According to the DOC website's published reports, in the whole year of 2008, there were only 270 foreign born inmates admitted to the MA DOC as a court commitment, of which only a fraction of 270 are illegal. Even if there were a roll-over of illegal alien inmates from 2007, it would not be many, as the average length of stay is 241 days. And even if this 806 figure did include county jails, we can't conclude costs of over $30 million using DOC figures; the inmate costs vary widely between each county jail and the DOC.

In short, Rep. Poirier raised many good points in her guest column on Secure Communities, but she made common mistakes in calculating state costs, a mistake that grossly overestimates actual costs to taxpayers.

PAUL HEROUX of Attleboro is a contributing columnist. He is a former director of research and planning for the state Department of Corrections. He can be reached at