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Data and Statistics

  • Sample Size And Volatility In School Accountability Systems

    Written on February 24, 2015

    It is generally well-known that sample size has an important effect on measurement and, therefore, incentives in test-based school accountability systems.

    Within a given class or school, for example, there may be students who are sick on testing day, or get distracted by a noisy peer, or just have a bad day. Larger samples attenuate the degree to which unusual results among individual students (or classes) can influence results overall. In addition, schools draw their students from a population (e.g., a neighborhood). Even if the characteristics of the neighborhood from which the students come stay relatively stable, the pool of students entering the school (or tested sample) can vary substantially from one year to the next, particularly when that pool is small.

    Classes and schools tend to be quite small, and test scores vary far more between- than within-student (i.e., over time). As a result, testing results often exhibit a great deal of nonpersistent variation (Kane and Staiger 2002). In other words, much of the differences in test scores between schools, and over time, is fleeting, and this problem is particularly pronounced in smaller schools. One very simple, though not original, way to illustrate this relationship is to compare the results for smaller and larger schools.

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  • What You Need To Know About Misleading Education Graphs, In Two Graphs

    Written on September 25, 2014

    There’s no reason why insisting on proper causal inference can’t be fun.

    A weeks ago, ASCD published a policy brief (thanks to Chad Aldeman for flagging it), the purpose of which is to argue that it is “grossly misleading” to make a “direct connection” between nations’ test scores and their economic strength.

    On the one hand, it’s implausible to assert that better educated nations aren’t stronger economically. On the other hand, I can certainly respect the argument that test scores are an imperfect, incomplete measure, and the doomsday rhetoric can sometimes get out of control.

    In any case, though, the primary piece of evidence put forth in the brief was the eye-catching graph below, which presented trends in NAEP versus those in U.S. GDP and productivity.

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  • Teachers And Education Reform, On A Need To Know Basis

    Written on July 1, 2014

    A couple of weeks ago, the website Vox.com published an article entitled, “11 facts about U.S. teachers and schools that put the education reform debate in context." The article, in the wake of the Vergara decision, is supposed to provide readers with the “basic facts” about the current education reform environment, with a particular emphasis on teachers. Most of the 11 facts are based on descriptive statistics.

    Vox advertises itself as a source of accessible, essential, summary information -- what you "need to know" -- for people interested in a topic but not necessarily well-versed in it. Right off the bat, let me say that this is an extraordinarily difficult task, and in constructing lists such as this one, there’s no way to please everyone (I’ve read a couple of Vox’s education articles and they were okay).

    That said, someone sent me this particular list, and it’s pretty good overall, especially since it does not reflect overt advocacy for given policy positions, as so many of these types of lists do. But I was compelled to comment on it. I want to say that I did this to make some lofty point about the strengths and weaknesses of data and statistics packaged for consumption by the general public. It would, however, be more accurate to say that I started doing it and just couldn't stop. In any case, here’s a little supplemental discussion of each of the 11 items:

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