Traditionally, a Carnegie Unit equalled an hour of instruction or study in a particular subject. American schools are typically in session for 180 days each school year, so a Carnegie Credit (at its original conception) equalled 180 hours of instruction or study.
Over the years many things have factored into a reconfiguring of what a Carnegie Credit means - so much so that it is fair to say that the Carnegie Unit really no longer exists, but the system does provide a framework by which a homeschooling family can easily figure credits according to time spent on task in a particular subject.
So starting with the idea that the Carnegie Unit equals 1 hour and a Carnegie Credit equals 180 hours we begin.
In today's "traditional" classroom, an hour is really not 60 minutes. Instructional time (which we'll call a classroom hour) can be anywhere from 45 - 50 minutes long depending on the school, the district and even the state. We'll go with a 50-minute hour because that's easy enough.
So here we go....
To figure how many 60-minute hours one should count for a credit, you take 180 times 50 (which, again, is how long an actual class-hour really is) and divide that number (9000 minutes) by 60. This tells you how many real hours you need to count. So in a traditional school that uses Carnegie units to figure credits, students are realistically only putting in 150 60-minute hours a year. To follow the letter of the Carnegie "law" you only need to account for 150 hours.
But of course, traditional schools apply flexibility to their formulas for figuring a credit hour, so it should not daunt the homeschooler to consider the many variations by which this 150 hours can be accounted for. But before you get too hung up on accounting for 150 hours, let's take a closer look.
You can consider that those 50-minute hours are not ALL really spent on task every day. Most classes need to take attendance and have some amount of time devoted on a regular basis to typical administrative tasks. After reading yesterday's newspaper I can see that every teacher of every class has to spend at least a few seconds telling students they need to put their i-pods and cell-phones away. You might also consider that most classes have at least one student asking an impertinent question or two or contributing to the discussion in such a way as to distract the class from the task at hand. Let's face it, discipline in the classroom probably costs at least a few minutes of many class periods.
You can think of all sorts of other scenarios and most of us, if we are honest, would have to conclude that there is no way that most classes are spent on-task for the full 50 minutes they are scheduled. And that doesn't even account for movie days or days when an unprepared substitute fills in and little to nothing productive happens.
SO, if you want to be generous and figure that traditionally-schooled students are conscientiously spending a minimum of 40-45 minutes each day in each class devoted to academics, you can do the math above to see that 120-135 hours/year/subject is closer to the what a typical public highschool student is offered in instruction (or teacher-contact time). Notice I didn't say it was actually productive time, it's simply instructional time offered for the taking by whatever students choose to do so.
Let me be very clear here before going on. I am not suggesting that a traditional classroom and the actual amount of time spent "on task" there is any kind of standard for a homeschooler. My point is to help the homeschooler gain some perspective and freedom in how the whole concept of credits may be viewed. My hope is that homeschoolers can become less concerned about time spent on task, and more concerned about the process and quality of learning that is actually going on.
But since this is a post about how a homeschooler might use the Carnegie credit system (which is supposed to be a non-subjective system for granting credit to secondary school students), let's look at that.
If you, as a homeschooler, are counting actual instruction/contact/work time, you ought to be able to recognize that your student's on-task time is probably much more productive than the average public school student's on-task time. And you should feel some freedom and flexibility in what you count toward credit. Personally, when I'm figuring credits based on time spent, I do a rough computation of how much time my sons spend on task, and at the same time weigh how proficient they have become in the subject matter they were to have covered. Again, putting in the time is not the goal. Learning is.
And on that note, I award credit where credit is due. The Carnegie system for figuring credits has evolved into something much more subjective than it originally was intended to be. In fact, the Carnegie system was designed to streamline the accounting process and remove subjectivity regarding evaluating whether a student was worthy to be given credit for work completed.
Subjectivity is unavoidable and I'm good with that. The goal is gaining an education, but colleges need a simple way to judge whether or not an education was got. While the credit system has become subjective to some degree, if you're wanting to grant credit according to time spent on-task with a particular subject the above explanation will hopefully help you see your way to doing so.
Elsewhere here I have written about various other ways to legitimately grant high school credit to your home schooled student. You will, undoubtedly, want to make use of more than one system for granting credit - quite simply because homeschooling provides opportunity for a variety of ways to achieve an education. Let your system of accounting reflect that.
Below are links to some excellent examples for public speaking students to model their final assignment after. Note the differences in Sarris's and Camillera's style in telling the same story. Both are telling the stories directly from scripture, but Camillera embellishes or uses more modern words at times. John Walsh simply retells the Bible story in his own words.