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8/19/2019 _7b954b527e0a4cc29d258a07559d9252_DICT-sec3_Holbrook

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Masking

Janet Holbrook, PhD, MPHJohns Hopkins University

Copyright © 2013 Johns Hopkins University and Janet Holbrook. All Rights Reserved.



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Outline

! Masking

-

Definition

-

Rationale

- Advantage and disadvantage



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Masking

! Synonymous with blinding

! Treatment assignment is not known after randomization

-

Patient, clinic personnel, evaluators, data processors, analyst,independent review committee, sponsor . . .

-

Single, double, triple, quadruple . . .

! Not the same as concealment of treatment assignment prior to

randomization

- Concealment protects against selection bias



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Rationale

! To reduce bias related to prior knowledge or beliefs about

treatment effects on the performance of the trial and on the

reporting of outcomes

!

Promotes objectivity in:

- Data reporting

- Data collection and follow-up

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Concomitant treatments, testing, behaviors

- Outcome assessment

-

Data interpretation



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Level of Masking

! Single

-

Participant is masked

! Double

- Participant and clinical investigator are masked

!

Triple

- Participant and clinical investigator and “others” are masked

! Others: outcome evaluators, data analysts, datamonitoring committees, sponsors . . .



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Advantages/Disadvantages

! Advantages of masking

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Protects against performance/reporting bias

! Data collection and follow-up

! Outcome assessment and reporting

!

Other care received during the trial

! Interpretation of results

! Disadvantages

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May be logistically or ethically impossible

- May not reflect clinical care practices

!

Concomitant care

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Increases logistical complexity-

Increased cost



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Bias protection

Level of Masking

None Single Double Triple

Data reporting No Yes Yes Yes

Data collection/follow-up No No Yes Yes

Concomitant care, testing,behaviors

NoNo/No/

YesYes Yes

Outcome assessment No No Yes Yes

Data analysis/interpretation No No No Yes

Bias Protection by Level of Masking



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Decision to Mask Treatments

! Is it ethical?

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Exposure to risk with placebo treatment, e.g., sham surgery,IV infusions

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Risk increased if investigator/caregiver doesn’t know

- Viable unmasking plan

! Is it possible?

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Distinctive characteristic, behavioral intervention, communityinterventions may make it impossible

-

Can you make the treatments seem “identical”

Or

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Can you create a placebo for each treatment, frequently

called a “dummy” treatment



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Decision to Mask Treatments (cont.)

! Trial design features

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Type of outcomes, more important for subjective ones

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Will outcome evaluation be masked regardless?

- What are the comparison groups?

!

No treatment

! Active control

! Is it feasible?

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Cost-benefit

- Is it practical? Would participants adhere to it?

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Effectiveness of masking



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Unmasking—Planned

! Strategy depends on closeout design

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Common closeout: all participants finish at “same” time

! Closeout visit or phone call

- Anniversary closeout: participants finish at different times

!

Should not be revealed to clinic staff

! Letter to participant at the exit interview

! Instructions for ways to continue treatment

!

Collect data on participant and clinic guess as to treatmentassignment before you reveal it

! Need to document, provide primary care physician with information



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Unmasking—Unplanned

! Discouraged—usually not required to treat patient

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Stop drug for adverse events

! Extenuating circumstances

- Next step in treatment may be more problematic

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Overdoses

! Easy access without being too easy

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Weigh risks

!

Methods

- On study drug container

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At clinics, semi-independent person responsible

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Contact parties responsible for randomizing/masking-

24-hour call line

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Web site

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