Table 1 Domains and risk of bias according to Fowkes and Fulton

Study design appropriate to objectives?

Objective common design

The type of study was marked in the appropriate type of study. If the type of study was appropriate according to the study design, it was labelled as "0", and as "++" if it was not appropriate

Prevalence Cross-sectional

Prognosis Cohort

Treatment Controlled trial

Cause Cohort, case–control, cross-sectional

Study sample representative?

Source of sample

The domain was considered [0] in cases of detailed origin, [+] to a specified origin of only one group and [++] in cases of absence of specification of the source of the groups

Sampling method

The item was assigned [0] for a full description of sampling method, [+] for poor or no explanation of sample method, with no problem in matching between groups, and [++] for poor or no description of sample method, interfering in the matching of the groups

Sample size

A minor problem [+] was considered when the sample was not representative or did not report a sample calculation. To a major problem, [++] was considered when no sample calculation was provided, and the number of participants was less than 50 participants, [0] was considered in the absence of the above factors

Entry criteria/exclusion

A minor problem [+] was attributed when the control and case group reported current use of antibiotics or anti-inflammatories, diabetes, smoking or pregnancy. In the case of presence of more than two previously mentioned items, it was considered as a major problem [++]

Non-respondents

The [0] was attributed when there was no refusal to participate in the study, [+] was assigned when there was the refusal, but did not compromise the sample, and [++] when there were refusal and impairment of the sample size

Control group acceptable?

Definition of controls

It was attributed [0] when all characteristics of the control group were described, [+] when any information was pendent as the origin of the control group, the selection criteria and a different origin between case and control groups and [++] when two or more items described in previously items

Source of controls

It was considered [0] when the control group was referred, [+] when the origin of groups was different, but with reasons and [++] when the groups presented different origins without reasons

Matching/randomization

In this item, [0] was assigned to cases of randomized/matched groups, [+] to cases of no description of randomization, but with a matching of groups and [++] to no explanation of randomization or matching

Comparable characteristics

It was attributed [0] to matched groups or not matched by the impossibility of being subsequently adjusted and [++] the presence of unpaired variables that were not paired or adjusted

Quality of measurements and outcomes?

Validity

It was considered [0] when the evaluation method applied is appropriate; [+] when using a single method, but with appropriate sensitivity with good specificity; [++] when using a single method, without an adequate specificity or good sensitivity

Reproducibility

It was considered [0] whether the evaluation methods were well described; [+] when a lack description of any step of the method was presented, for example, the identification of the patients of the groups studied in laboratory samples, evaluations at different times or application of various methods between groups of individual pathology; [++] when two or more of the previous items are present

Blindness

The condition of the study participants was considered to be "Blind," in this case being assigned the signal [0], in cases of "not blind" the signal [++] was attributed

Quality control

It was considered a problem when the examiner was not qualified; a partial periodontal exam was performed [not in all teeth or not in all the six periodontal sites/teeth], the measurement of periodontitis was only radiographic or the absence of the number of evaluated teeth sites. A Minor problem [+] was considered when 2 of these characteristics were present, and a major problem [++] if more than 2 of these characteristics were present

Completeness

Compliance

It was assigned [0] for a sample size that remains the same from the beginning to the end or decreases without compromising the power of the test; [+] for differences in sample size at the end of the study, compromising the power of the test, but with reasons and adjusts; [++] for difference in sample size at the end of the study, compromising the power of the test, without reasons

Dropouts

The [0] was scored when there is no loss during the study, [+] when there is a withdrawal that involves the inclusion criteria, such as age, sex, [++] when there is withdrawal and it compromises more than one criterion

Deaths

This item was scored as Not Applicable [NA], due to the type of PECO strategy

Missing data

In this item, [0] was assigned to cases of randomized/matched groups, [+] to cases of no description of randomization, but with a matching of groups and [++] to no description of randomization or matching

Distorting influences?

Extraneous treatments

In this item, [0] was considered when there were no external influences; [+] when there are external influences, but that does not interfere in the results; [++] when there are external influences and interferes with the results

Contamination

This item was scored as Not Applicable [NA], due to the type of PECO strategy

Changes over time

In this item, [0] was attributed to data collected in the same period; [+] to data obtained from the control group and the study group at different times that may cause distortions; [++] when the previous item was associated with data from studies already published

Confounding factors

A problem was assigned when the data analysis involved enrollment of persons < 5 years. Menopausal woman, smokers, diabetics and obese. A minor problem [+] was assigned when 1 or 2 of these characteristics were present and a major problem [++] if there were 3 or more

Distortion reduced by analysis

It was considered [0] when it cites the adjustments of the covariates that present distortions; [+] when the article report adjustment, but does not say the criteria; [++] when distortion was identified, without adjustment

Summary questions

Bias: Are the results erroneously biased in a certain direction?

YES or "NO" answers were assigned to each question. If the answer is NO to the three questions, the article is considered reliable, with low risk of bias

Confounding: Are there any serious confusing or other distorting influences?

Chance: Is it likely that the results occurred by chance?