Where endogenous pests - e.g. weeds, nematodes, soil borne diseases can build up from crop to crop and pest management action taken this year affects the level of attack in subsequent years, the costs and benefits of a pest management strategy need to be calculated over several years. In this case, an investment model is the appropriate tool for making an economic assessment. Since there will be various costs and benefits over time, how does one make a comparison. The trick is to express all of the future costs and benefits in terms of their present value by applying a discount rate, allowing present and future values to be compared.