Grade level stratification or GLS is our term for sorting educational content by grade level. Because different states teach the same content in different grades there is no simple way to do GLS. There is really no such thing as the grade level of a document per se. It all depends on the state and school it is to be used in.

To meet this challenge we have developed a new statistical approach to GLS that seems to work well. Each document is assigned a grade level based on the grade levels of the science concepts it uses or teaches. The more advanced the concepts the more advanced the document.

As for the science behind our approach to GLS, below is a brief summary. See also http://www.stemed.info/reports/STEM_Edu_Center_GLS_report.htm for the evolution of our method.

We begin by identifying the grade level of many of the words and phrases commonly taught in science education. These terms are not just vocabulary, rather they represent the core concepts being taught. A concept is what one has to know in order to use a term properly, so concepts are bodies of core knowledge that are attached to core terminology.

Our GLS grade level is the estimated National Average Grade Level or NAGL (rhymes with bagel) for each term. The NAGL is a STATISTIC, not an actual grade. There is a lot of state to state variation in when each term is taught (but not the order, that is relatively fixed by concept dependencies). The variation is due to "spiraling," which is jumping back and forth between different disciplines.

For more on spiraling see "A 3-D Spiral Model of K-12 Science Education".

Most terms are within a 3 to 4 grade range between states, but very few are taught in the same grade in all states. It is really quite a mess.

The K-12 NAGL for each term is presently estimated by averaging the grade levels found in three state standards -- VA, CA & TX. These three standards are our sample data as to when each term is taught nationally. The college NAGLs are estimated using textbooks.

The K-12 terms are extracted as follows. First we find a term that actually occurs in the standards, and the grade level for each state. If a term occurs more than once in a standard we find the earliest grade, because that is when it is taught. We then average the states to get the NAGL for that term. Then we find variants and closely related terms that are likely to be taught at the same time. (This is oversimplified because what occurs in different standards are often variants or related terms, not the same terms.)

The GLS-based grade of a document is then based on the grade level of its most advanced concepts. These are the concepts that the document is probably explaining or teaching. Existing documents can be sorted by GLS and new documents can be written using GLS.

Note that many, perhaps most, users of GLS will be learning these terms in actual grades other than the NAGL. Averages are like that. This is going to cause some confusion until people understand how to use NAGL-based GLS. In any case the NAGL gets the user to documents that are close to their grade and eliminates a lot of higher and lower grade content, so it works.

Perhaps the easiest way to understand GLS is to look at some stratified term lists. Here are several early versions of our master lists. They are for illustrative purposes only.

1. Master list: Space science -- K-12 (LL 1-8) terms.

2. Master list: Space science -- basic college (LL 9) terms