To interpolate a series, the cells between the first and last cells in the series must be blank. Interlocation is the process of producing a time series at a higher frequency than is actually available, for example a monthly seres from quarterly data. To calculate a series of values from a starting point without a defined ending point, you have to extrapolate the values. When you extrapolate a series, the step value represents the amount that each value is increased or multiplied as the series is extended.