What is rate? How do you calculate it? What does it tell you? (contributed by Sarah Coughlin)

• Remember that rate is always expressed as something changing over time. This means that to calculate a rate, you must divide the change in the y-axis by the change in the x-axis. In the example below, the rate that Speedy is running could be calculated as: 10km -2km = 8km = 3.2 km/hr 3hr - 0.5 hr 2.5 hr Another way to calculate rate would be to insert a trendline and equation onto the graph (see description of trendline below). In the example below the equation of the trendline is given in the graph: y = 3.3367x + 0.4121. This equation is in the format y = mx + b. The 'm' of the equation, or the 3.3367 in our equation, is equivalent to the slope of the trendline. The slope of the trendline is equivalent to the rate. This 3.3367 km/hr is slightly higher than our 3.2 km/hr we calculated above. This is because the trendline is a best-fit line, a line that is drawn that best represents all the points. A trendline can be used to predict things by extending the line forwards or backwards. Because our points are not all in a straight line, this will be slightly off our hand calculated slope, however, the Excel equation of the trendline is actually more representative of the slope than our calculations were. This means that it is not only easier to use the computer to calcuate a rate, but it is more accurate. (If you have problems remembering how Excel works, look on page R2.4 in your lab manual.)