(18)A distinction should be made within the category of small institutions as some are very small, while others are close to the maximum thresholds allowing them to benefit from the simplified system. Under a single lump-sum system, the annual contributions of very small institutions would be disproportionately higher than those of the small institutions which are close to reaching the maximum thresholds. At the same time, it should be avoided that the simplified system leads to a disproportionate difference in terms of annual contributions between the biggest among small institutions and the institutions which do not qualify for the simplified system as they are just above the thresholds. To avoid such unwanted effects it is therefore appropriate to foresee a system of several categories of small institutions whose annual contributions should consist of different lump sum amounts. This should allow for a progression of contributions within the simplified system, and between the highest lump sum and the lowest contribution pursuant to the method whereby the basic annual contribution is adjusted according to the risk profile of the institution.