 So, the average rate's all very well, but sometimes you want to look more precisely at a rate, and for this we use instantaneous rate. Whereas the average rate is the average gradient between two points, the instantaneous rate of a reaction is equal to the slope of a line that's tangent to the curve at that point. We write this as a differential using a small d instead of delta. If you've done calculus in maths you'll recognise this notation. If you haven't, the idea is not complicated. In the expression for average rate you're calculating the average gradient between two specified points. Now imagine bringing those two points closer and closer together until the distance between them is infinitesimally small. This is effectively the same as calculating the gradient of the tangent to a single point, and this is what the small d means. So we have average rate measured over some finite interval of time, and we have instantaneous rate which is the rate at a particular instant in time. So which of these are you looking for when you do an experiment? Well ideally you want instantaneous rate. This is the most accurate way of looking at the rate of reaction. However sometimes, for a variety of reasons, the data isn't good enough to give you an instantaneous rate. In such a case you use the average rate, but you try to make the time intervals that you take your measurements at as small as possible. More frequently you measure your data, the better the picture you'll have of how the rate is changing. With some reactions you can't measure anything until the reaction is finished. For instance if you're waiting for a specific colour change like in the iodine clock reaction. In this case you're forced to measure an average rate across the whole time of the reaction. This is fine and it can still give you good data. Final note. In investigations the most important part of your data is the initial rate. This is the rate at the moment that the reaction starts, the starting speed if you like. Make sure you collect good data right at the beginning and you can then look at the effect on this initial rate of some variable like the concentration of a reactant or the temperature of the reaction.