Were the groups similar at baseline?

Ideally, the patients in the treatment and control groups should be identical in every respect except for the intervention or therapy that is to be studied. This is not always possible but the paper should detail the similarities and try to statistically adjust for differences.

Application to Pollock et al's thyroxine paper

Patients showed a trend to being heavier and having somewhat deficient throid function as compared to controls. All were significantly different in some psychological measurements from the controls. However this was a crossover study so the patients acted as controls.

Table A Baseline characteristics of patients with symptoms of hypothyroidism but thyroid function tests within the reference range and for controls. Values are means (SD) except for numbers

     
    Patients (n=25)
    Controls (n=19)
    No of men/women
    2/23
    2/17
    Age (years)
    47.6 (11.4)
    48.9 (10.4)
    Previous thyroxine treatment (n)
    3
    0
    Previous psychiatric contact (n)
    6
    1
    Referral:
    General practitioner (n)
    13
    NA
    Self referral (n)
    9
    NA
    Other (n)
    3
    NA
    Free thyroxine (pmol/l)
    13.3 (2.1)
    14.2 (2.1)
    Free triiodothyronine (pmol/l)
    3.0 (0.67)
    3.4 (0.60)
    Thyroid stimulating hormone (mU/l) at 0 mins
    1.9 (1.1)
    1.4 (0.9)
    Thyroid stimulating hormone(mU/l) at 20 mins
    14.6 (9.5)
    13.1 (6.9)
    Cholesterol (mmol/l)
    6.3 (1.3)
    5.2 (0.86)
    Prolactin (mU/l)
    312 (199)
    233 (116)
    Ferritin (ng/l)
    61 (43)
    57 (54)
    Antithyroid peroxidase antibodies (U/ml)
    184 (450)
    32 (58)
    Weight (kg)
    85 (20)
    67 (12)
    Pulse (beats/min)
    79 (15)
    72 (11)
    Diastolic blood pressure (mm Hg)
    81 (12)
    77 (9)

    NA=not applicable.


 

Table B Scores on psychological assessment measures at baseline for patients with symptoms of hypothyroidism but thyroid function tests within the reference range and for controls. Values are means (SD)

     
    Patients
    (n=25)
    Controls
    (n=19)
    P value
    Cognitive functioning
    Logical memory I
    18.68 (5.88)
    29.11 (6.03)
    <0.001
    Logical memory II
    15.09 (6.58)
    25.63 (7.60)
    <0.001
    Verbal paired associates I
    15.86 (3.54)
    19.00 (3.00)
    NS
    Verbal paired associates II
    6.45 (1.34)
    6.95 (1.18)
    NS
    Visual reproduction I
    30.00 (6.58)
    33.89 (3.81)
    NS
    Visual reproduction II
    25.32 (7.07)
    31.74 (5.82)
    NS
    Digits forward
    8.55 (1.57)
    9.00 (2.31)
    NS
    Digits backward
    6.95 (2.30)
    8.11 (2.79)
    NS
    Trail making test
    97.50 (39.97)
    61.68 (16.99)
    0.001
    Psychological functioning and physical health
    Hospital anxiety and depression scale
    20.91 (8.74)
    8.37 (5.04)
    <0.001
    SF-36 health survey:   
     
     
    Role emotional
    33.33 (44.84)
    82.45 (35.78)
    <0.001
    Physical functioning
    35.91 (30.69)
    87.50 (18.62)
    <0.001
    Role physical
    13.64 (30.06)
    84.21 (26.63)
    <0.001
    General health
    37.50 (24.10)
    84.79 (14.08)
    <0.001
    Vitality
    26.60 (22.06)
    64.47 (17.79)
    <0.001


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