While these data represent the most comprehensive account of the condition of sites designated for nature conservation in the United Kingdom yet produced, a number of provisos need to be made.
Because the guidance on feature objectives was made available progressively during the six-year monitoring period, not all assessments were made against objectives formulated in accordance with this guidance.  This issue will reduce in significance as the monitoring programme is rolled forward. 
Ideally results would be presented both by numbers of features and by area of features.  This would allow biases caused by large numbers of small features (such as tend to occur in the lowlands), or a few large features (the corollary in the uplands), to be avoided.  Unfortunately, it is not possible at the current time to present data by area of feature, and all the graphs presented are therefore for numbers of features.
The four statutory nature conservation agencies carrying out the monitoring undertook this work in a manner determined by priorities in their various countries.  For example, the Countryside Council for Wales (CCW) concentrated their effort on SACs, plus some assessments on SPAs.  The reports from English Nature (now Known as Natural England)concentrated on habitat SSSIs and SACs; there are fewer reports relating to features on species SSSIs and few reports on SPA or Ramsar features.  Such discrepancies between countries can be expected to reduce as the monitoring programme is rolled forward.
If a feature is reported in unfavourable condition it does not mean that the whole of that feature is degraded.  The nature of the assessments made, and the use of several attributes for each feature, means that it may have failed on one attribute rather than several, and may have done so by a small margin rather than a large one.  It is important, therefore, to understand that the threshold for unfavourable condition is set within guidelines on a site by site basis.  This is essential to ensure that site management is focussed on the local circumstances in which a feature is found.
Quality assurance of procedures was undertaken by the individual nature conservation agencies in accordance with processes determined by the agency concerned.  Further quality assurance to ensure comparability of assessments within and between agencies will be introduced shortly.
It is possible that the method treats SAC features more severely than SSSI features.  SACs tend to be larger than SSSIs and are often an aggregation of SSSIs.  Aggregation rules for data mean that parts of a feature assessed as unfavourable can cause the whole feature to be assessed as unfavourable.  This is more likely to happen on aggregated or large sites.  Another factor maybe that SAC features are selected because they are particularly in need of conservation across Europe.  In addition, SAC features tend to be defined more tightly than A/SSSI features and therefore targets are set more precisely.