"cannot be directly tested" is a correct and usable phrase in written English.
You would use it when something cannot be measured or checked with a concrete test or method. For example, "The claim that our customer service is the best in the industry cannot be directly tested, but customer reviews on our website suggest otherwise."
This assumption cannot be directly tested since the information on the counterfactual is not available.
This cannot be directly tested if assignment is based on more covariates than the researcher observes, because then the marginal treatment recipient is not identified.
This exogeneity condition cannot be directly tested due to missing unbiased estimates for (varepsilon_{in}) and requires making a judgement based on personal knowledge and common sense.
While this assumption cannot be directly tested, it implies similar distribution of individual characteristics on both sides of the eligibility threshold.
In this work, only adult individuals were studied and thus this hypothesis cannot be directly tested.
Currently a role of cortactin at the NMJ in vivo cannot be directly tested because suitable models – viable animals with global or muscle-specific cortactin gene deletion – are unavailable.
Since the "true" coevolutionary history of a protein cannot be experimentally determined, measures of coevolution cannot currently be directly tested.
Current models predict that a Robo1 receptor that cannot bind Slit should not be able to repel axons in vivo, but this prediction has not been directly tested.
This hypothesis has never been directly tested.
The assumption itself thus cannot be directly put to test with the approach of this study.
The data analysis method employed is structural equation modeling (SEM), a second-generation statistical tool efficient in modeling latent variables (those that cannot be directly measured) and testing the complex relationships among the variables [ 12, 14].
