Here is the design of a clinical trial:

Official Title: Randomized, Double-blind, Placebo Controlled, Superiority Phase II Study to Evaluate the Safety, Pharmacokinetic, Efficacy of Gabapentin as add-on to Morphine in Children From 3 Months to Less Than 18 Years

Included conditions:
- Chronic Pain

Study Armgroups:
- {'label': 'Group 1', 'type': 'EXPERIMENTAL', 'description': 'Gabapentin + Morphine', 'interventionNames': ['Drug: Gabapentin + Morphine']}
- {'label': 'Group 2', 'type': 'PLACEBO_COMPARATOR', 'description': 'Placebo + Morphine', 'interventionNames': ['Drug: Placebo + Morphine']}

Interventions:
- {'type': 'DRUG', 'name': 'Gabapentin + Morphine', 'description': 'Gabapentin: liquid oral formulation (syrup) - 75 mg/ml-three times daily. The starting dose during the optimization period will be defined according to 2 weight groups (5-15kg and \\>15kg). Dose will be scaled at Day 1,3,5,14,and 21 according to a specific schedule. Maintenance dosing will be scheduled in accordance to the body weight.\n\nMorphine: background therapy as oral, liquid and solid formulations. Patients with BW≤30kg :liquid oral formulation four times daily throughout the whole treatment period. Patients with BM\\>30kg : immediate release solid and/or liquid oral formulation four times daily during titration phase and an extended release solid oral formulation twice daily during the maintenance period.', 'armGroupLabels': ['Group 1']}
- {'type': 'DRUG', 'name': 'Placebo + Morphine', 'description': 'Placebo liquid oral formulation. Morphine: background therapy as oral, liquid and solid formulations. Patients with BW≤30kg :liquid oral formulation four times daily throughout the whole treatment period. Patients with BM\\>30kg : immediate release solid and/or liquid oral formulation four times daily during titration phase and an extended release solid oral formulation twice daily during the maintenance period.', 'armGroupLabels': ['Group 2']}

Primary Outcomes:
- {'measure': 'Pain scores', 'description': 'Pain scores in the two treatment groups assessed at baseline and at the end of the treatment by age-appropriate pain scales. FLACC (the Faces, Legs, Arms, Cry and Consolability - pts aged 3-24 months) scale composed by 5 categories. Each category is scored on the 0-2 scale, which results in a total score of 0-10, where 0=Relaxed and comfortable, 4-6=Moderate pain, 1-3=Mild discomfort, 7-10=Severe discomfort or pain or both.\n\nFPS-R scale (Faces Pain Scale - Revised - pts aged 3-7 years): score is associated with face 0, 2, 4, 6, 8, or 10, where 0 = no pain and 10=very much pain.\n\nNRS-11(Numerical Rating Scale - pts aged 8-17 years): numerical rating scale where 0=no pain and 10=worst possible pain', 'timeFrame': 'on average of 16 weeks'}

Please estimate the sample size based on the assumption: 
A two-tailed test with a significance level of 0.5, a power of 80%, and 10% dropout rate.
Sample size calculation
          A total sample size of 66 is needed. It will allow detection of a significant difference between morphine plus placebo and morphine plus gabapentin groups in mean pain scores at EOS. Mean baseline pain scores of 8.5 are assumed for both groups, with estimated EOS pain scores of 5.4 for morphine alone versus 4.5 for morphine plus gabapentin based on percentage change in pain scores in a previous trial in adults [31].
          A minimum number of patients is defined per age group: (1) at least 5 patients aged 3âmonths to less than 3âyears, (2) at least 15 patients aged 3âyears to less than 7âyears, and (3) at least 20 patients aged 7âyears to less than 18âyears. No maximum number of patients per age group is specified. A two-tailed test will be applied with a significance level of 0.5 and a power of 80%, and 10% dropout is anticipated.