Table 4 Criteria used for critical appraisal: global sources of biases assessed for internal validity of studies included in our systematic review (I/E: Intervention/Exposure)

Confounding bias

Error of assessment in the association between I/E and outcome (a factor is independently associated with the I/E and the outcome)

Presence of a control before and/or without I/E

Randomization (e.g., randomized allocation of animals in groups), stratification and statistical adjustment

Selection bias

Difference of composition between control and I/E groups

Same numbers of study objects in control and I/E groups

Similar baseline for I/E and control groups

Measures to avoid contamination between I/E and control groups

Performance bias

Error of assessment between the effects of the I/E and the consequences of confounding variables

Control of confounding variables that particularly contribute to the spatial and temporal heterogeneities, by repeating the measurement

Adaptation of the study duration (sufficiently long to see the effect of I/E)

Detection bias

Difference between groups in how outcomes are determined

Error of assessment between the effects of I/E and the consequences of measurement errors

Representative sampling: composite samples

Same sampling times for comparison between I/E and control groups

Control of conditions to sample, transport and store samples to limit their degradation and external contamination

Analytical replication and control

Attrition bias

Difference between initial and final groups (at the end of study), with respect to trial outflows, treatment interruptions, loss of samples

No real solution; it would be appreciated if authors justify the potential attrition in the course of the study

Reporting bias

Incomplete reporting of materials and methods or outcome results