The SS uses a highly controlled lab setting, which is likely to produce behaviours which are unlike those in a natural setting.
If the artificial environment is producing artificial behaviour, then the SS is not a valid measure of attachment types.
Evaluation for the SS- weakness:
The Strange Situation has poor ethics.
The procedure is designed to cause distress & 20% of infants are described as 'crying desperately' when assessed.
Many psychologists say that we should not rely upon the SS to assess attachment type, if it is not ethical.
Evaluation for the SS- strength:
Good inter-rater reliability.
As the Strange Situation relies on groups of observers, there needs to be an agreement in their ratings.
Ainsworth found a correlation of +0.94 between raters.
We are more able to draw firm conclusions from reliable results.
Evaluation for the SS- weakness:
The Strange Situation is culturally biased.
The procedure is heavily biased towards an American viewpoint of what 'normal' attachment looks like, however, other cultures may rear their children differently, which isn't acknowledged within this measurement (links to cultural variations).
This reduces the validity of the SS & therefore its generalisability across cultures.