So that means what ? ....

The easiest way to understand how to use the factor-label or dimensional analysis method is by example. Lets take a couple examples that have nothing to do with nursing ...

Suppose we're driving along on the way to visit Judy with the cruise set at 64 miles per hour. We have Bob in the passenger seat. Bob asks, "we've been driving for 2 hours - how far have we gone?". Now of course, we all know how to solve this without using any of the factor-label stuff I'm talking about - and that's also why it's a good example to see how it works ...

We start with what we want to know, that is distance, or miles.

 

 

What else do we know?
Two more things: how fast we're going and for how long.

(our speed)  64 mi/hr and  2 hr (how long).

 

 

Since what we want to know has miles as the numerator, to set up the problem we take the piece of information we know that has miles in it and write it so miles is in the numerator :

 

 

!! This is essential !! We have to make sure we orient a "known" factor the same as what we want to know.

And then we multiply by the other thing we know - how long - writing it so its units (hr) are opposite out first factors units :

 

 

Now we just go through and cancel and multiply :

 

 

This seems pretty trivial - but it demonstrates the "trick" of using the factor-label method: Start with what you want to know and make sure your first factor has the same unit in the same position. Then add what ever other factors you need until the only unit(s) remaining after canceling corresponds to the unit(s) of what you want to know.

As a rule of thumb, your first factor will ALWAYS have the unit in the SAME position. Every additional factor will be oriented with a unit in the OPPOSITE position.

Now you may be wondering, how does this tie into that ONE formula for doing dose calculations? The "what we want to know part is obvious". How about "ordered" ? In the information we have available, the "directive" we have is, "drive for 2 hours". That's our "order". How about available dose & available form? These 2 things always have to do with how we deliver the order or how we carry the order out. What we have available to deliver distance is our speed. The available dose is 64 miles and the available form is per 1 hour.

©1997-2006 Dale Sampson, RN