Statistical design is a key factor in impact evaluation. It helps to define the sampling scheme, the sample design, the selection and how to expand the results obtained in a sample to the whole range of properties studied (expansion factors).

It is recommended that this design be developed by a specialist, however the aspects that the designer should consider when choosing the sampling type required should be highlighted; this will facilitate dialogue with the specialist and the subsequent interpretation of the data.

1 Confidence and precision parameters should be chosen; for this type of study the recommended confidence level should be 95% and never less than 90%. The precision or margin for error should not exceed 5%. As the error rate is limited, the sample tends to be greater and collection more costly.

2 .The sampling technique should be chosen: the basic technique is simple random sampling; although this is easy to calculate, it tends to generate very large samples that increase the cost of the evaluation. Stratified sampling is recommended for LAPs, establishing homogeneous subgroups within the sample based on territory, in order to have greater precision. For LAPs at least three groups should be set up for the strata:

This stratification is the key to the analysis because the behaviour of the variables may be expected to be very different in urban and rural areas, particularly in relation to land use, property value and perception of security.

3. Take into account how the projects are implemented. LAPs have had two types of intervention which can have consequences for the effects of RCT processes:

• Systematic regularization of a whole geographic area, consisting of cadastral mapping, legal diagnosis and the systematic regularization of tenure in parcels that meet the eligibility criteria of the programme.
• Voluntary regularization, which offers facilities for individuals or groups of individuals who are applying to the programme for regularization processes.

The effects of voluntary regularization may be greater than those of systematic regularization because in the former case beneficiary households are those which have shown personal motivation to obtain a title or other forms of strengthening their tenure rights. It is therefore more likely that they will try to obtain certain types of benefits once the tenure of their land has been regularized (see bibliography for further information).

These two types can in turn form two study groups as follows:

These three groups would apply equally to urban or rural cases. In cases where an evaluation is performed when the regularization process has not been completed, a further subdivision could be applied to the intervention group with systematic regularization, as follows: