From Policy Concepts in 1000 Words: Success and Failure (Evaluation):
Policy success is in the eye of the beholder. The evaluation of success is political in several ways. It can be party political, when election campaigns focus on the record of the incumbent government. Policy decisions produce winners and losers, prompting disputes about success between actors with different aims. Evaluation can be political in subtler but as-important ways, involving scientific disputes about:

How long we wait to evaluate.
How well-resourced our evaluation should be.
The best way to measure and explain outcomes.
The ‘benchmarks’ to use – should we compare outcomes with the past or other countries?
How we can separate the effect of policy from other causes, in a complex world where randomised-controlled-trials are often difficult to use.

In this more technical-looking discussion, the trade-off is between the selection of a large mixture of measures that are hard to work with, or a small number of measures that are handpicked and represent no more than crude proxies for success.
Evaluation is political because we set the agenda with the measures we use, by prompting a focus on some aims at the expense of others. A classic example is the aim to reduce healthcare waiting times, which represent a small part of health service activity but generate disproportionate attention and action, partly because outcomes are relatively visible and easy to measure. Many policies are implemented and evaluated using such proxies: the government publishes targets to provide an expectation of implementer behaviour; and, regulatory bodies exist to monitor compliance.
Let’s consider success in terms of the aims of the person responsible for the policy. It raises four interesting issues:

The aims of that policymaker may not be clear. For example, they may not say why they made particular choices, they may have many reasons, their reasons may not be specific enough to be meaningful, and/or they may not be entirely truthful.
Policymaking is a group effort, which magnifies the problem of identifying a single, clear, aim.
Aims are not necessarily noble. Marsh and McConnell describe three types. Process measures success in terms of its popularity among particular groups and its ease of passage through the legislature. Political describes its effect on the government’s popularity. Programmatic describes its implementation in terms of original aims, its effect in terms of intended outcomes, and the extent to which it represented an ‘efficient use of resources’. Elected policymakers may justify their actions in programmatic terms, but be more concerned with politics and process. Or, their aims may be unambitious. We could identify success in their terms but still feel that major problems remain unsolved.
Responsibility is a slippery concept. In a Westminster system, we may hold ministers to be ultimately responsible but, in practice, responsibility is shared with a range of people in various types and levels of government. In multi-level political systems, responsibility may be shared with several elected bodies with their own mandates and claims to pursue distinctive aims.

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