Here is the design of a clinical trial:

Official Title: Ultrasound Guided Superficial Cervical Plexus Block for Analgesia After Craniotomy Via Suboccipital Retrosigmoid Approach: A Randomized Controlled Trial

Included conditions:
- Plexus Block;Analgesia;Neurosurgery

Study Armgroups:
- {'label': 'Superficial cervical plexus block group', 'type': 'EXPERIMENTAL', 'interventionNames': ['Procedure: Superficial cervical plexus block']}
- {'label': 'Control group', 'type': 'SHAM_COMPARATOR', 'interventionNames': ['Other: Control group']}

Interventions:
- {'type': 'PROCEDURE', 'name': 'Superficial cervical plexus block', 'description': 'superficial cervical plexus nerve block will be performed under the guidance of ultrasound', 'armGroupLabels': ['Superficial cervical plexus block group']}
- {'type': 'OTHER', 'name': 'Control group', 'description': 'ultrasound guidance will be used to determine the location of superficial cervical plexus nerve. The puncture will also be performed by ultrasound guidance, covered with opaque infusion dressing but performed with infusion of 10 ml normal saline.', 'armGroupLabels': ['Control group']}

Primary Outcomes:
- {'measure': 'the cumulative consumption of sufentanil by the PCA', 'description': 'the cumulative consumption of sufentanil by the PCA 24 hours after surgery', 'timeFrame': '24 hours after surgery'}

Please estimate the sample size based on the assumption: 
The sample size calculation assumes a two-tailed significance level of 0.05, a power of 90%, and a dropout rate of 10%.
Sample size calculation
        We estimate the sample size according to the primary outcome of postoperative-24-hour PCIA sufentanil consumption by using PASS 2011 software (NCSS LLC). Based on the previous literature,21 Akcil et al demonstrated the mean (95% CI) postoperative cumulative morphine consumption was 30 mg (25 to 35) in the scalp block group and 50 mg (40 to 60) in the control group. Considering that 1 mg morphine is equivalent to 1 Âµg sufentanil, we estimated the scalp block in their study reduced PCIA sufentanil consumption by 20 Âµg within postoperative 24 hours. In the routine practice without SCPB, we also apply PCIA for the patients undergoing craniotomy via suboccipital retrosigmoid approach with the dosage of sufentanil as 50 Âµg during the first 24 hours after surgery. So, we estimated the effect size of mean as 20 Âµg with the SD of 30 Âµg for the SCPB group comparing with the control group. The sample size of 106 patients will be sufficient to detect the difference at a two-tailed significant level of 0.05 and a power of 90% using Studentâs t-test, with a drop-out rate of 10%.