Your theory of change (see evaluation practice 1.2) enables you to visualise the issue in hand and identify required inputs and outputs, as well as expected outcomes and anticipated impact(s) all at the outset of your planning.
It is always advised to start with an evaluation strategy when planning any kind of initiative or intervention. Having an evaluation strategy gives visibility of the ‘pre’ intervention (or baseline data) you need to have available (or collect) in order to be able to evaluate the impact of your intervention i.e. what ‘pre’ and ‘post’ intervention data you need to have available to capture (any) impact. See evaluation practice 4.6 and practice 4.7 for discussion on types of data you might collect according to your intervention. Evaluation undertaken as a ‘late addition’ to the project, rather than being planned ahead will leave you unsure or unable to establish some of the impact of your intervention.
Having identified the intended outputs, outcomes and impact it is then a matter of determining the data and evidence (see evaluation practice 4.6) that will demonstrate if the desired outputs, outcomes and impact have been achieved. Where possible this should draw on existing data sets available within the organisation (see guidance in evaluation practice 4.7). So there are choices to be made here informed by your theory of change.
At this stage, and the focus of this evaluation practice, ethical issues including General Data Protection Regulation (GDPR) compliance as embodied in the Data Protection Act 2018, and organisational issues will need to be considered and you will likely need to revisit these once the scope and evaluation criteria, questions and methods have been determined (see again evaluation practice 4.6 and practice 4.7).